A rebar cage is the reinforcing steel for a piece, tied or welded together into a single assembly before it goes in the mold. Concrete is strong in compression but weak in tension, so the cage carries the tension the piece will see in service.
The shop drawing specifies the bar sizes, spacing, and cover, and the cage has to match it. Building cages is its own step in the plant, and having them ready is often what gates whether a piece can be poured on schedule.