As they speed up, the air resistance increases until the two forces are balanced, so they eventually reach a maximum speed and don’t go any faster. This is their terminal velocity.

The problem is that air resistance depends on the velocity of the object. Search your feelings, you know this to be true. When you are driving (or riding) in a car and you stick your hand out the ...

Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity.