Barrier

Barriers are one of the perks that come up on level 15. They are basically walls and can be used to make mini bases, or to block incoming bullets when you are getting shot.

Countering
To counter someone with a barrier, wait until they have used their barrier, then start attacking them. They can't use the barrier until the cooldown stops, which can allow you to do some damage. Unless they have already made a wall of barrriers, they are still very open to attack. It is quite hard to destroy a barrier (snipers needing 3-4 hits despite players needing 2-3 and assault guns doing almost zero damage), so unless they are completely surrounded by barriers, it is not a good idea to destroy one.

Using
To use a barrier, press space. It's very useful to clutch with it, because if you time it properly, they can block a bullet. Snipers, assault rifles, and shotguns can actually shoot through walls from any team. It's a bug where the gun's end goes past the wall if you move into the wall as much as you can and the bullet ends up going through the wall. It's a really nice defense technique, though a lot of people use it.