A bond or other loan that must be repaid comes due on its maturity date. On that date, the full face value of the bond (and sometimes the final interest payment) must be paid in full to the bondholder.
Certificates of deposit (CDs) also have maturity dates on which you may withdraw the principal and interest without penalty or roll over the money into a new CD.
Other debt instruments, such as mortgage-backed securities, pay back their principal over the life of the debt, similar to the way a mortgage is amortized, or paid down. While these instruments also have a maturity date, that date is when the last installment payment of the loan as well as the last interest payment is due.