Joint and several liability means every tenant who signs the lease is on the hook for all of it. If three roommates split a $3,000 rent informally at $1,000 each and one stops paying, the landlord can pursue any or all of the tenants for the full shortfall — the roommates' private split is their own arrangement, invisible to the lease. Most standard residential leases with multiple tenants are written this way, usually through an explicit "jointly and severally liable" clause.
For landlords, the clause is basic protection: one lease, one rent, and no obligation to referee who owes what among roommates. For tenants, it is the fine print that surprises people — a departing roommate does not automatically shed liability, and a co-signer or guarantor typically stands behind the whole obligation on the same joint-and-several basis. How the clause is enforced, and what happens when one co-tenant wants out mid-lease, involves state law and the lease's own terms, so treat the general principle here as a starting point rather than the rule in your jurisdiction.
This is general educational information, not legal or tax advice. Rules vary by state and locality and change over time — check your local law and confirm specifics with a qualified professional.