For flat renting, what does "Not HMO licensed" mean?

A property with HMO, or Housing in Multiple Occupancy, licence means the landlord is allowed to rent out the flat to 3 or more unrelated people (usually students or professionals). A flat with an HMO licence must meet certain requirements, such as fire safety, etc. It is illegal for a landlord to let their flat to 3 or more unrelated people without that flat having an HMO licence, and the landlord is fined if discovered by the council.