It means that should the payee of the note or subsequent holders in due course note elect to extend the term of the note, with or without consideration it shall not affect their rights, remedies or liens secured by the note.
As you may know all endorsee’s on a promissory note are liable to subsequent holders in due course.
For example if A gives his promissory note to B for $1000 and B then endorses it over to C and C to D and D to E. Then each A, B, C and D are liable to E for its payment.
That particular provision in the note means that if E agrees with A to extend the term of the note, it does not release those subsequent endorsers of their ultimate obligation to honor the note.
There could be circumstances without that waiver provision whereby an intervening endorser would be released from being obligated to the subsequent holder, say E, if E’s actions in extending the term of the note were to prejudice a prior endorser’s
rights to seek reimbursement.