What is the difference between a bi-plane and mono-plane?

A biplane has two (bi) sets of wings, and a monoplane has one (mono) set of wings.

The two sets of wings on a biplane add lift, and also drag, allowing it to fly slower. The one set of wings on a monoplane do not add as much lift or drag, making it faster, and as a result, all the fastest planes are monoplanes, and most planes these days are monoplanes.