A versatile weapon can be used with one or two hands, and has different damage values depending on which. Versatile weapons always deal more damage when wielded with two hands (one size die bigger, to be exact). However, this prevents you from using a shield with the weapon, or wielding a different weapon in your offhand.

In the Player’s Handbook, Versatile is listed as a weapon property, followed by a parantheses including a damage die. This damage die in parantheses always refers to the weapon’s two-handed damage. The damage type is the same as the weapon’s normal damage.