Getting max'd out is when you have enough power in either one or several fleets of ships that it reaches the limit in which the game allows....
9.223.372.036.854.775.808 power to be exact.
To achieve this you need an obscene amount of credits, I think several quint (don't quote me on this as I've never got anywhere near achieving this)
Once you reach this power you cannot attack or be attacked by other players.
....and on the main server you have to be Australian aswell!!!
(thats just a wee jest at a couple of players!!!)