Sure you have heard these terms, but applied eg meteorology and other sciences.
The best description of these terms is when applied to the Topography.
All mountains have two sides, the side which receives air currents or winds, is the windward side (exposed to the wind).
The other side is the Leeward side (they are covered or protected from the wind).
The windward side is usually wet (Moisture bring precisely the winds).
The leeward side is usually dry (There are no winds that bring moisture.
Therefore, the windward side has abundant vegetation, while Leeward, vegetation is sparse.
