A sublease is usually a lease contract between a tenant and a subtenant. The subtenant inherits the lease terms the tenant signed.
As a landlord, you can write it in the lease that you prohibit subletting. However, some cities do have ordinances that prevent the subletting prohibition from being enforced. If you choose to allow subletting, it's important to also be a part of the process. Some of the things you can do include:
If you're not a fan of subletting, you can also allow the tenant to break their lease early for an early termination fee. This fee can cover the vacancy risk as well as the costs of finding a new tenant.