This is best shown by way of the following example. The landlord owns commercial premises and the tenant is leasing the premises for twelve years. In year six, the tenant decides to slim down their business activities, take a break or sublet the premises at a higher rent than his rent to the landlord.
The tenant needs to sublet part of the term of years left on the lease and becomes the sub-lessor. To be able to do this, the tenant must get formal consent from the landlord in the form of a Licence to sublet, which is signed by the landlord, the out-going tenant and the in-coming tenant.
The head landlord remains safe as the new tenant (the 'sub-lessee') is liable to the old tenant (the 'sub-lessor') and old tenant remains liable to the head landlord.