It's when you abandon/transfer your rights to someone else.
Let's say you have a contract with an attorney and your relationship turns sour.
By a contractual waiver you accept to go to a jury trial to solve the dispute.
I hope it's clear enough..
Your first sentence is right, but I'm afraid the rest isn't, FranParis.
Waiving one's rights has two meanings.
The first is simply that one forbears from enforcing the rights. For example, if somebody owes you money and you never insist that they repay you, you have waived your right.
The other is that you agree, as part of a consensual variation of the contract, not to rely on a right that the contract gave you. Usually, the waiver is in return for another benefit granted by the other party to the contract. In other words, he has "bought" the waiver from you.
The second is the better meaning of a "contractual waiver". the first is a voluntary waiver.
But neither meaning has anything to do with going to trial, whether before a jury or in any other way. In fact, it means the opposite, because the consensual variation prevents any dispute.