The term All-in (also "Allin" or "All in") usually refers to the committing of or to a single attack which can yield a huge gain or a potentially devastating loss.
All-ins can arise from different game situations: they can either be planned (such as a rush build), used for breaking a Wall-in, or simply be forced, i.e.; as an desperate attempt to win the game if an attack has decimated your Economy.
Verbally speaking, the term All-in can be used as "to be all-in" meaning that the player is performing an All-in move.