A sublease (or sublet) is when a tenant rents their unit — or a room in it — to someone else while the original lease stays in force. The original tenant becomes a kind of in-between landlord: they usually remain responsible to the property owner for the rent and the condition of the unit, even though a subtenant is living there.
Whether subletting is allowed generally depends on the lease and on local law; many leases require the landlord’s written consent first. Spelling out the rules up front — and documenting any approved sublease — prevents most disputes over who owes what.