What is a double block in volleyball?

Answer 1

A double block is when two players are working in together to deflect an attacked ball at the net.
When 2 players jump up and block together. Usually the middle and outside or the middle and setter.

Answer 2

A block is used by a front row player. It is when the other team is hitting the ball from the front row and then the player from your team throws up his/her arms in an attempt to keep the other team from making a kill.