You create an invisible magical force which can be used in a number of ways;

Pushing/pulling an object 5' away from its current position, inflicting 1d4 dmg, DC 13 DEX

Creating a spherical force which appears and protects you. Until the start of your next turn, you have a +1 bonus to AC, including against the triggering attack, and you take half damage from magic missile.

Other examples include but are not limiting to the creativity of the Caster; 

Knocking an object out of someone's hand, DC 13 DEX to hold on. Triggering a trap, creating bubbles underwater, shattering a glass, closing a book, throwing a rock, disarming an opponent as a reaction, slamming a door, etc.