It's the command Apply, with its shortcut @@:
And@@{cond1,cond2,...}
Structurally, Apply[f,g[x1,x2,...]] removes g and replaces it with f. In our case, List[cond1, cond2, ...] becomes And[cond1, cond2, ...].
Apply[f,g[x1,x2,...]]
List[cond1, cond2, ...]
And[cond1, cond2, ...]
