Power creep is the gradual unbalancing of a game due to successive releases of new content. As new expansions or updates are released, new game mechanics or effects are introduced making it increasingly difficult for older content to remain in balance without changes. Usually, this means new content releases grow successively more powerful (hence "power creep") while older content becomes relatively underpowered.
In Guild Wars (and other computer games), developers are able to release updates which alter older content to try to keep balance. ArenaNet performs major rebalancing after the end of each tournament season.