(1) Not connected. For example, all printers have a switch that allows you to turn them offline. While the printer is offline, you can perform certain commands like advancing the paper (form feed), but you cannot print documents sent from the computer. The opposite of offline is online.
(2) Aside from its technical meaning, offline is used frequently in a more general sense to describe events that occur outside of a standard procedure. For example, if somebody at a meeting says "let's continue this discussion offline," it means "let's discuss it informally at another time."
The term is alternatively spelled as off-line.