Up ahead, a pile of cubes and other stuff will be pushed out of the way. Go forward and GlaDOS will tell you about aerial faith plates.

There is a greenish target on the ground, but you can ignore that for now. In front of the target, there is a clear platform on the ground with a little green dot on it. This is the aerial faith plate. Stepping on it will catapult you forward. So step on it.

You will land on a target on the other side of the room. There is a red button in front of you. Press it to make a cube fall down in the middle of the room, behind you. A cube will be bounced up and down on another aerial faith plate.

Turn around to face the bouncing cube, wait for the cube to be close to the floor, then step onto the aerial faith plate here. If you timed it correctly, you should be able to press the Use Item button (check your settings if you don't remember which button you assigned to the Use Item action) to grab the cube as you fly past.

With the cube in hand, use the aerial faith plate on the other side of the room to jump with the cube to the exit. Put the cube onto the big red button, then go through the exit door and into the elevator.