Joint liability is when you have a financial responsibility to cover any rent or damage payments that may be owed by your housemate - or they in turn may have to cover money that you owe. So if you or anyone else moves out early and owes rent, the landlord can chase the person who owes the money or can choose to collectively ask the household to pay.
You will have joint liability if you:-
Sign the same housing contract as your housemates at the same time and you are agreeing to take the property as a group.
However, you will not have joint liability if you move in later than everyone else (maybe as a replacement tenant) and/or your name is just added to the contract.
Joint liability also means that if someone moves out and wants a replacement tenant to move in, the rest of the household have a right to say whether they agree with the replacement tenant moving in. However, remember that if the household don’t agree, joint liability means the remaining tenants could end up paying towards any rent owed by the person moving out - if that person can show the remaining tenants are not being reasonable.