What it means
Joint and several liability means each of several parties is responsible both together and individually for the full amount of an obligation. The person owed money can pursue any one of them for the entire sum, not just that party's share. In Australia this commonly appears in contracts, guarantees and partnerships. A party who pays more than their fair share may then seek contribution from the others, but the claimant doesn't have to chase everyone — they can recover the lot from whoever is easiest to enforce against.
How it's used
Because the two guarantors were jointly and severally liable, the bank recovered the entire loan from just one of them.