If you're referring to an Offline Map in Google Maps, then it's just that: a map (or portion of it, more accurately) that's been saved to your phone for offline access. That way, you can store certain map areas so that you can access them even if your phone is not connected, and since GPS doesn't need a network connection, you can see your current location on a map anyway. VERY useful if you're traveling abroad and want to avoid roaming data charges but still want to know where you are and how to move around.
Just remember, offline maps are not available in all regions (some countries impose restrictions as do some areas and/or states, disabling offline mode for those) and that you cannot search for a particular address or do turn-by-turn navigation with offline maps; for those features you do need an active connection.