An autosave is an automated saving of the game. In single-player, this is done whenever the module builder deems a save is appropriate (such as shortly before a particularly challenging encounter). In multiplayer, this can be set to be done at regular (timed) intervals. This can help avoid losing too much playing time should things go rather solidly against the player(s) best interests.
There is only one slot reserved for autosaves (slot number 1). This means that a second autosave will overwrite the first. In order to keep multiple saved games, the game must be manually saved. When selecting a saved game to load, the autosave is listed near the top of the list of saved games, alongside the quicksave, above the named manual saves.