A System update is when Jagex launches one or more of their updates to the main game engine. Updates are typically content-related; e.g. a new quest, a tweak/improvement to a recently released quest, or a rework of an existing section of game code. Players will have to reload the game window in order to log back in once the update takes effect. In addition to that, all NPCs (Shooting Star / Evil Tree / Penguins / Etc) and Game Objects (Rocks / Trees / Etc ) will respawn. This makes for a good time to train.