When two or more parties are jointly and severally liable for a tortious act, each party is independently liable for the full extent of the injuries stemming from the tortious act. Thus, if a plaintiff wins a money judgment against the parties collectively, the plaintiff may collect the full value of the judgment from any one of them. That party may then seek contribution from the other wrong-doers.
For example suppose that A, B, and C negligently injure V. V successfully sues A, B, and C, for $1,000,000. If the court used a joint and several liability system, V could demand that A pay V the full $1,000,000. A could them demand contribution from B and C. However, if B or C could not pay, A would be stuck paying the full $1,000,000.
Joint and several liability reduces plaintiffs' risk that one or more defendants are judgment-proof by shifting that risk onto the other defendants. Only if all defendants are judgment-proof will a plaintiff be unable to recover anything. However, this system can cause inequities, particularly where a relatively blameless defendant is forced to bear the financial burden of an incredibly guilty co-defendant's insolvency.