Attacking With Spell:
Some spells automatically hit. For example, Magic Missle automatically hits the target, and the caster rolls the damage dice listed.
Other spells either have an Attack Roll:
CASTER’S Ability Modifier + CASTER’S Proficiency + CASTER’S 1d20 roll
Or other spells might have a Chance to Save:
TARGET Ability Modifier + TARGET’s 1d20
The target’s total must be equal to or higher than the caster’s Spell DC (Ability Modifier + Proficiency + 8)
Spell Ability is the modifier from whichever stat your class is focused on (Int, Wis, or Cha). Example: A wizard’s spell ability is Int, so the spell ability modifier is the + bonus from his int (+2, for example).