This is brain-dead simple.

Let's suppose there's a contract, and it has some clause or provision in it.

Then let's suppose Party A ignores this provision and thus is technically in violation of it.

But then Party B ignores that Party A is ignoring the provision and is in technical violation of the clause, Party B doing nothing about this state of affairs.

The clause or provision then becomes null and void because, even though it's technically in the contract, both parties are ignoring it, behaving in actuality as if it doesn't exist. Their actual course of conduct over-rides and replaces the provision, since obviously neither really cares about it. Their course of conduct becomes the new contract.

This concept is designed to prevent contracts from being littered with meaningless provisions that can be used sometime down the line by one party or the other only as trap doors against the other party.

Needless to say, there's a common problem here with HOA's, where new people are always coming in promising to "crack down" on the rules -- rules which no longer legally exist -- because they think something put in writing sometime in the distant past, something that no one really cares about any longer, is as permanent as if it had been carved in stone like the mythical tablets of Moses.