Floor Joists. They are like jacks. There are several websites out there that can give you more info.
first determine where the floor sags. Lay a 6ft. or longer level across the floor in sections, you must then discover if the floor is accessible from below. if it is re support the floor joists and while you are there apply project liquid nails to any gaps in flooring and joists. Now, after you have gotten the floor fairly level; and depending on what type of floor is going to go down, use leveling thin set to take out the uneven spots. good luck with your project.
