This is best shown by way of the following example.
A landlord owns a commercial premises and their tenant is leasing the premises for 12 years. In year 6 of the lease, the tenant decides they want to sublet the business premises because they want to slim down their business activities, take a break or sublet for a higher rent than the landlord’s rent.
The tenant decides to sublet for a part of the term of years left on the lease and now becomes the ‘sub-lessor’. To be able to do this, the out-going tenant must get formal consent from the landlord in the form of a Licence to sublet. This is signed by the landlord, the outgoing tenant and the incoming tenant.
The head landlord remains safe as the new tenant (the 'sub-lessee') is liable to the outgoing tenant (the 'sub-lessor') and the outgoing tenant remains liable to the head landlord.