The idea of reducibility is really just a matter of using a solution of one problem as a subroutine in solving another.
We say problem Y is (polynomial) reducible to X if we can solve Y using a polynomial number of calls to an algorithm for X.
This implies that Y is as easy as X (X is at least as hard as Y).
We used this idea when we talked about network flow and linear programming: we can reduce network flow to linear programming very directly.
