The melee attack rolls you make are not the only times a character swings their weapon in a round. There are feints, parries, and other movements that are not rolled. As such, if someone drops their guard, they open themselves up to attack. Such an attack is called an opportunity attack. You can use your reaction to attempt an opportunity attack when a hostile creature that you can see moves out of your reach. An opportunity attack is a single melee attack against the provoking creature made immediately before the creature leaves your reach. The Disengage action can be used to avoid provoking opportunity attacks. Movement through teleportation does not provoke opportunity attacks, and unless noted otherwise neither does forced movement (such as if you are pushed away by a creature or effect, or if you move past a creature while falling).