When your build step starts, you may choose one of your Yuuzhan Vong in this arena. If you do, then chosen unit gets Stealth and "This unit's speed is 40, and this unit can't gain or lose speed." until end of turn.
Keywords
Stealth
Stealth: An isolated, static ability that means, "This unit can't be attacked unless it's tapped or has damage counters on it." An untapped and undamaged unit with Stealth can still be affected by alternative types of damage. (Such as Overkill.) As long as the unit has any number of damage counters on it, it can't use Stealth.