An HMO (house in multiple occupation) is a property that is rented out by 3 or more people who do not form a household (i.e. are not a family). Bathroom and kitchen facilities are shared between residents.
A large HMO is one where 5 or more people rent out the property. Large HMOs need to meet certain standards and obligations, and require a license from the local council.
For further information or to apply for this licence, visit the gov.uk website.