You measured and said I have a "high joist". What is that?

Joists are like the bones of your home. They are structural pieces of wood that hold it all together. A joist is a horizontal, structural member used in framing to span an open space, often between beams, that subsequently transfer loads to vertical members. In your home these are covered up by your subfloor which is typically 3/4" oriented strand board. When one of the joists sits higher than the others this creates a rise in the floor that will slope down on both sides of the joist (like a broken or bent bone).  This uneven plane must be corrected in order to install many flooring products to meet manufacturers specifications of flatness.