"To butter" means some version to "apply butter to". So "to butter the bread" means to "put butter on the bread" or "spread butter on the bread."
You could also "butter toast," "butter English muffins," or "butter the pan for cooking."
"To butter up" means "to praise or flatter someone, usually in exchange for gaining favor in some sort of way."
She buttered up her boss to get the promotion.
He buttered up the gate agent in attempt to get a better seat on the plane.
They buttered up the teacher in hopes she would allow the exam to be open-book.
He buttered up the instructor so that she would let him into the class, even though it was full.
"Butter up" is a phrasal verb that can be split.
Don't even try to butter him up because he doesn't value that kind of flattery.
Just butter the parking attendant up a little bit and he won't make you pay for parking.