A more thorough explanation:

An undertenant is a person who rents a property from a sublessee. In other words, an undertenant is a sub-sub-tenant. They are not the original tenant who signed the lease with the landlord, but rather someone who is renting from the sublessee.

John signed a lease with the landlord to rent an apartment for a year. However, after six months, John needed to move out of town for work. Instead of breaking the lease, John found someone else to take over the lease and become the sublessee. The sublessee, Sarah, then rented out one of the rooms to her friend, Emily. Emily is now the undertenant, as she is renting from Sarah, who is renting from John.

This example illustrates how an undertenant is someone who is not the original tenant, but rather someone who is renting from a sublessee. In this case, Emily is not the tenant who signed the lease with the landlord, but rather someone who is renting from the sublessee, Sarah.