That is the number used to calculate how your spell is powered and how powerful it is. Each Caster uses its own modifier; Wizards and Artificers use their Intelligence modifier. Druids, Clerics, and Rangers use their Wisdom modifier. Sorcerers, Paladins, and Warlocks use their Charisma modifier. To get this modifier, you get your stat, subtract ten, then divide by two. It would look like this ([Stat-10]/2) and end up with a number between zero and six.