[F22Rapture]

I think more important than describing FIRST, even, is outlining your leadership and involvement in the team. Remember - a resume should focus on you and what makes you qualified, so if you can say "FIRST is this, and we did this" that's good, but if you can say "As elected team leader / lead programmer / head of inventory, I was responsible for X, Y and Z" that's much better. You want to convince them that you're a team player who can get along with others, while also being capable of making decisions and putting forth your own ideas.

With that said,

"FIRST is an international robotics competition in which teams of high-school students from across the world are given 6 weeks to design, build, and program a functional robot which is able to fulfill the unique and complex challenges specific to that year's game."

[ your responsibilities and leadership roles, especially elected positions ]