This is best shown by way of the following example. The landlord owns commercial premises and his tenant is leasing the premises for twelve years. In year six, the tenant decides to close his business and no longer wants the premises.
The tenant needs to transfer or assign to another business the rest of the term of years left on the lease, otherwise he’ll have to pay the rent for another six years. The tenant needs permission to assign the lease and this is given by way of a Licence to assign signed by the landlord, the out-going tenant and the in-coming tenant.
Note you’ll then need a deed of assignment to actually implement the transfer of the lease.