A break clause allows you and your landlord to end the contract early by giving written notice. If the break clause allows just your landlord to end early, the clause could be unfair and therefore invalid. If you have a joint tenancy, you will all need to agree to use the break clause and leave early.
If you have a break clause you will need to give written notice at least 2 months before the break clause date. For example, if you have a 12 month contract with a 6 month break clause, you may be able to leave after 6 months, but you will need to give 2 months notice (so 4 months after the start of the contract).