Solid Hardwood is 100% hardwood milled from lumber. As a natural material, hardwood reacts to changes in its setting, like moisture and extreme temperatures, which cause solid wood to shrink or expand. All solid hardwood can be sanded and refinished if needed over the years. Solid hardwood is not recommended for installation below ground level or in bathrooms.
As the name implies, this type of wood flooring is solid wood all the way through, from top to bottom. Solid hardwood floors require a wood subfloor. It's similar to any other stick of lumber that sawn straight off of the log--no additional materials added. It's usually just called "hardwood floor."
Hardwood does require sanding, staining (optional), and sealing after installation. You need to limit usage until it has been sealed. After sealing, you will need to wait at least 48 hours for the sealant to dry. Even then, additional coats may be required. The advantage is you can stain it and seal it to your exact specification.