Floor joists are horizontal structural members used to support floors in buildings. They are typically made of wood, steel, or engineered materials and are spaced evenly apart to create a sturdy framework. The joists help distribute the weight of the floor and any furniture or occupants above it, ensuring stability and safety. These joists are usually installed between foundation walls or beams and are covered with subflooring to create a solid surface. Proper installation and spacing of floor joists are crucial for preventing sagging and maintaining the overall integrity of the building structure.