The Marathon rest stage is a stage in Super Monkey Ball: Step & Roll, which is exclusive to Marathon mode. It is a level with no goal and no time limit.
Description[]
The stage is a giant Wii Balance Board with red guardrails around the perimeter. There are eight bananas around the perimeter for the player to collect. Jam is shown reclining on a regular sized Wii Balance Board next to a water bottle and some headphones.

Jam taking a rest on a smaller Balance Board
Role in Marathon[]
In Marathon mode, after completing all of the levels in any given world, the player is transported here. They can spend as much time here as they like and can collect the eight bananas for additional points. The player can continue with their marathon by pressing A. This level presumably exists as a courtesy to those using the Wii Balance Board to play the game as they may get tired from prolonged usage of this control method; and there is no down time between levels in Marathon mode.