"when in doubt throw it out"

What does the phrase "when in doubt throw it out" means?

Pretty much what it says: if you're not sure if that milk in the fridge smells a little off or not, don't take chances, discard it. Got a boyfriend who's been mean to you, but you're not sure about his promises to reform? Ditch him. Do there seem to be, too, many, commas in that sentence? Edit them out.