Stu & Pid are obstacles in SMG3's lair to prevent Mario and SMG4 from stopping him from uploading a video containing content of SMG4's weirdest and most embarassing moments.
Stu & Pid are Piranha Plants who seem to be brothers, and as their combined name suggests, they are StuPid. They constantly fight over small things and call each other dumb, which is, in fact, true, again relating to their combined name.
Stu is a yellow Piranha Plant who says the lie while Pid is purple and he says the truth