The part command has you leave a room. The other people in the room will see the message if you send one. The example below would make you leave the room #webmaster.
Example:
/part #webmaster See you later.
You don't need to use a part message, but many people like to use it either to say good-bye or to include a small personal quote, much like an email signature. Parting a room has you leave that room, but remain on the chat network and in any other rooms you might be in.