Sitting there like a stuffed lemon" means to remain in a place or situation in a completely passive, useless, and inactive way, without taking any action. The phrase combines the idea of a useless, defective "lemon" (a slang term for something unsatisfactory) with the image of a stuffed, immobile object, emphasizing a complete lack of engagement. It is often used as a criticism or an instruction to start being helpful.
You could've googled this yourself instead of sitting there like a stuffed lemon