I assume that you’re talking about what’s on the sides of the arms. Those are rubber bands and are available from vex, although I think that most teams buy them elsewhere. They’re really useful for helping your arm raise, often the motors will not be strong enough when you have 5 sacks or so in whatever mechanism you use to hold the sacks.
They are a very effective and low cost “Turbo” for your lift assembly! With 4 on each side we were able to lift several more sacks and have less staling. Like they said, make sure you buy the right size. We got a huge bag at staples and replace them before each tournament. You may also want to take them off when the robot is being stored for any length of time between matches.