The attack block prevents hostile attacking actions.
It will say something like "The attack block is active till dd.mm.yyyy hh:mm:ss. In that time only friendly fleets can be started."
Attack blocks (bans) seem to come into effect when some significant server maintenance event takes place, such as migrating to new servers. The ban is Universe wide OGame board.
It is not known if this also serves as some kind of cheat detection against automated attack bots but presumably it does not incorporate this functionality and serves only to facilitate the server changes.