Fee simple is full ownership of both land and building, as opposed to a leasehold interest.
Most single-tenant NNN properties are sold in fee simple, which means the buyer owns the land and the building and the lease is attached to the property. This differs from a ground lease or leasehold, where the investor owns only part of the interest. Fee simple ownership gives the owner the land value as a backstop, which supports resale and re-leasing options. Title insurance and a survey help confirm exactly what is being conveyed.