The target of a spell must be within the spell's range. For a spell like Magic Missile, the target is a creature. For a spell like Fireball, the target is the point in space where the ball of fire erupts.

Most spells have ranges expressed in feet. Some spells can target only a creature (including you) that you touch. Other spells, such as the Shield spell, affect only you. These spells have a range of self.

Spells that create cones or lines of effect that originate from you also have a range of self, indicating that the origin point of the spell's effect must be you (see "Areas of Effect").

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.