From the Super Mario Wiki
Ads keep the MarioWiki independent and free :)
Bowser's body is a location from Mario & Luigi: Bowser's Inside Story and its remake. After Bowser, under the influence of the Vacuum Shroom, inhales everyone at Princess Peach's meeting, Mario and Luigi must search for the missing princess within his body.
On the map of Bowser's body, the big areas with many enemies are denoted with green circles, places where minigames are played are red circles, places where moves are unlocked are blue circles, and finally the only other place, the challenge node, is yellow.