A waiver is the intentional and voluntary giving up of something. A waiver may be made by an express statement or by conduct, such as not enforcing a right. Some activities require a waiver of liability from participants, especially in dangerous activities such as scuba diving or car racing.
Revoking a waiver is difficult, but it has been successfully done in some states. A person would generally need to prove that there was some sort of trickery, coercion or fraud involved in the presentation of the waiver. One form of coercion might be that the waiver was presented to you too late for you to back out, effectively removing your right to decide if you should sign.
For example, a landlord may impose a late fee for failure to pay rent on time. However, if the landlord has made a practice of accepting late payments without charging the fee, he may be deemed to have waived the right to collect a late fee. In a further example, when a party fails to file an answer or make a court appearance in response to being served with a complaint, they are deemed to have waived the right to dispute the claims and a default judgment may be entered in favor of the plaintiff.