When your build step starts, you may choose an activated ability of one of your Thieves.
Beckett's Gang gets the chosen ability 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.