If you are familiar with the games that use spellcasting ability or attacks, it isn't much of a problem. But as a starter, it is essential to know these kinds of features beforehand. To better understand the spell casting modifier, you need to know what spells in a game are or how they are cast. Spellcasting ability as per Reddit is

'' Spellcasting ability is the ability you use to cast spells: Int, Wis, or Cha. Your class will specify which, under the Spellcasting class feature. You use that ability modifier to determine your spell attack bonus and spell save DC.''

Fantasy game like Dungeon & Dragon uses magical abilities in its characters to express multiple ways. One of the most common expressions of magic in the game is through spell casting. All kinds of characters, monsters and NPCs, all have distinctive ways of preparing the spells they could use. Every character can create and use them, but the question is; What are these spells, and why should we cast them?

What are spell and spell casting?

A spell is described as the simple shaping of the magical energy that fuses the multiverses into limited and straightforward expression. More simply, it is that a character is absorbing the raw energy around the character's surroundings and use that later according to desire.

These desired effects can include versatile tools like creating water, fireballs, damaging effects, shields, and many more. There are hundreds of spells that can be used in D&D; these are categorized in different ways.

First, the spells are dedicated levels up to nine; each level indicates how powerful or damaging the spell can be. You can see that the level of the spell is scaled along with the spell caster. The other way spells are cast by schools, the spells of the same group, or alchemy. There are the following schools in the game.

1. Abjuration

These are the spells that focus on protection.

2. Conjuration

Spells that transport objects and creatures.

3. Divination

These spells reveal information.

4. Enchantment

Spells that affect the minds of others.

5. Evocation

Spells that channel elemental or healing energies.

6. Illusion

Spells that affect the senses of others,

7. Necromancy

Spells that manipulate the energy of life and death.

8. Transmutation

Spells that change the properties of an object, creature, or environment.

All the spells fall in these categories, no matter how many a character knows. There is a limit to how many spells can be cast as it uses both the physical and mental power of the character. There is an allotted number of spells that a character can use at each level.

The higher the level, the more slots you have. It is essential to have spellcasting components like magic words, hand gestures, or material components like a particular object to cast a spell successfully. If you are unable to provide one or more components, you can't cast a spell. To launch a spell, a character must know the spell and have prepared expanded slots for that spell. All spells have a limited range and duration.

What are Spellcasting modifiers in D&D?

The spellcasting modifier depends upon its character's class. Spell attack modifier is your proficiency bonus added to your charisma modifier. Magic in Dungeon & Dragon is an integral of the game; it multitudes the very world of D&D and is expressed in various ways, from powerful beings with latent supernatural abilities to craft items and the artifacts of power. The most common expression of magic in the game is through the casting of a spell.

So, you have started playing D&D and have decided to be a spell caster; whether you're a divine caster like Cleric or an arcane caster like a Wizard, your spells are going to follow the same basic rules except for Warlocks. Now, before you can cast a spell bullet into your gun, you generally can't cast a spell you haven't prepared; it can still vary according to the classes. 

Your spellcaster modifier is what you use to add attack rolls with your spell. Your spellcasting modifier varies with the class as if you are a bard, Sorcerer, or Warlock, will use charisma. If you are Cleric or Ranger, you will use wisdom. Intelligence for the Wizard, Arcane Twister, or Eldritch Knight. Using a modifier increases the proficiency of the attack.

Cleric and Druid

Divine mages like Cleric and Druid have a wild array of spells. They always have access to and can choose which to prepare at the top of the day.  To modify their attacks,  Cleric uses their wisdom as a vital component.

Wizard

Arcane classes like the Wizard can only prepare spells they have scribed in their spellbook, but the collection can grow and grow into a massive library of options.

Bard or Sorcerer

These modifiers draw their magic power from within and have a much narrower number of chosen spells, but they have them prepared at all times.

With your spells prepared, you must keep track of your spell slots. Spells are mentally and physically taxing, so you can only cast a few a day. The more powerful they are, the fewer times you can cast. As we already know, they are set at levels from 1 to 9, 1 being the simplest and 9 the most powerful level. When you cast a spell, you must expand the spell slot that correlates with its power level. To understand in a simpler way, we can imagine that you are a Balthazar and are a 2^nd level cleric, which means you have three 1^st level slots. Your Barbarian go hurt because that's what he does so you can cast first level cure wounds on him, burning one of the precious slots, which means you have two more first level spell slots left until you need to take a long rest to replenish them. Being aware of the information discussed above can help you be a better gamer.