What is joint and several liability?

It means every tenant on the agreement is responsible for the whole rent, not just their share.

If four sharers are jointly and severally liable and one leaves without paying, you can pursue any of the others for the full amount. It is the standard arrangement for a shared house on one agreement.

The alternative is separate agreements per room, which is what room by room lettings use. That gives you cleaner replacement of one tenant but usually makes the property an HMO with everything that follows.