A subfloor is the structural layer that sits directly on top of the floor joists and provides a base for the finished flooring material. It's essentially the foundation of the floor structure in a building. The subfloor serves several important functions:

1. Support: The subfloor supports the weight of the occupants, furniture, and other loads placed on the floor.
2. Stability: It provides a stable and level surface for the installation of finished flooring materials.
3. Attachment: The subfloor serves as a base for attaching the finished flooring, such as hardwood, laminate, tile, or carpet.
4. Structural integrity: It contributes to the overall strength and stability of the building structure.

Subfloor materials can vary depending on factors such as building codes, climate, and budget. Common materials used for subflooring include plywood, oriented strand board (OSB), and traditional tongue-and-groove boards. The choice of subfloor material depends on factors such as the expected load, moisture resistance requirements, and compatibility with the finished flooring material.