A lease is a legal document, but can be an oral arrangement, which confers a right on one person (called a tenant or lessee) to possess property belonging to another person (called a landlord or lessor) to the exclusion of the owner landlord. The relationship between the tenant and the landlord is called a tenancy, and can be for a fixed or an indefinite period of time (called the term of the lease). The consideration for the lease is called rent or the rental.
Under normal circumstances, owners of property are at liberty to do what they want with their property, including dealing with it or handing over possession of the property to a tenant for a limited period of time. However, if an owner has surrendered possession to another (ie the tenant) then any interference with the quiet enjoyment of the property by the tenant in lawful possession is itself unlawful.
Similar principles apply to real property as well as to personal property, though the terminology would be different. Similar principles apply to sub-leasing, that is the leasing by a tenant in possession to a sub-tenant. The right to sub-lease can be expressly prohibited by the main lease.
