A mechanical pencil, or propelling pencil, is a refillable pencil that never needs sharpening. These pencils are made from metal or plastic (often both) and have a mechanism for pushing out the graphite ("lead") so you can draw with it. With traditional pencils, the pencil lead is revealed by removing the wood around it with a pencil sharpener.
Mechanical pencil leads come in various widths and degrees of hardness. Most pencils will take only one width or size of lead, but the hardness doesn't matter. Spare leads can be stored in the top of a mechanical pencil until needed.