A waiver of contract rights happens when a party gives up a right or chooses not to enforce it.
A waiver clause often says that a waiver must be in writing and that not enforcing a right once does not mean the right is lost forever. This helps avoid arguments that delay, silence or past tolerance changed the contract.
Waiver issues can be fact-specific, so the wording and conduct of the parties matter.
This answer is general information, not legal advice.