Answer:
Joint and several liability means that all the tenants are responsible for all the rent. So, if John and Betty both sign the lease, and the rent is $1,000, it's not like John owes $500 each month, and Betty owes $500 - they both owe the whole thing. The landlord can't collect $2,000 total, once he gets the $1,000, that's it for that month. But, if John pays $500, and Betty leaves town (permanently), and doesn't pay, the landlord could sue John for the remaining $500, since he now can't find Betty. John doesn't get to argue that he paid his half, and is off the hook. He is jointly and severally liable.