Plain English Meaning
Checking that the system which lets one wing's fuel feed the other side's engine is working properly before you might need to rely on it.
Full Definition
A preflight or ground verification that the airplane's fuel crossfeed system actually works as intended — confirming that fuel can be routed from a tank on one side of the airplane to feed an engine on the opposite side, and that the crossfeed valves, selectors, and associated plumbing operate correctly.
Context Anchor
You see this in multiengine airplane fuel-system procedures, especially before flight, during a run-up, after fuel-system maintenance, or when reviewing emergency fuel procedures.