There may be situations where you need to find another flatmate. The one you had moved out or you may need to find someone to help with the rent. Sub-letting is where you give another person the right to stay in your rented place while keeping the lease in your name.
In these situations you become a head-tenant with the responsibilities of a landlord when dealing with the other person who is a sub-tenant. As head-tenant you alone are responsible for paying the rent to the landlord and organising to have things repaired.
Before you sub-let you need the landlord's permission and they can refuse your request. If you don't let the landlord know that you are sub-letting then you are breaking the terms of the lease and you and/or the sub-tenants may be asked to leave.