Joint liability means that all parties involved are equally responsible for the obligations of a loan, debt, or other liability.
Being severally liable means that if one of the parties is unable to pay their portion of a debt, then the other parties in the agreement are responsible for their own portions as well as the unpaid portion.
Being both joint and severally liable at the same time means that all parties involved are equally responsible for the obligation of the contract (such as a loan), and that any one of them could be individually responsible for the entire obligation.
For example, if three people were to enter into a Loan Agreement as borrowers with joint and several liability, they would all be equally responsible for paying off the whole debt. If one of the three parties was unable to pay their share, the other two parties would become responsible for paying the entire balance.