This is the explicit throw of Motoko, which maps to issuing a “reject” on the system level. On the system level, calls have either a reply or a reject as their response.
Don’t confuse throw (aka reject) with the concept of a trap, which happens when you divide by zero. That is not catchable within WebAssembly. Mildly confusing, it often appears that a trap causes a reject, but that is not always the case, and in some sense a trap in the canister causes a the system to response with a reject on the canister’s behalf.