The short answer
A promise is generally enforceable only if each side gives something of value in exchange, called consideration. The rule matters most when a contract is changed or a promise is one-sided.
Where it fits in your case
When one side argues a promise or modification is unenforceable.
What to expect
Existing duties. Promising to do what you already owed usually is not new consideration.
Past work. A promise to pay for work already done is often unenforceable on its own.
Illusory promises. If one party can walk away at will, the deal may lack consideration.
Goods contracts. Under the UCC, good-faith modifications need no new consideration.
What we need from you
The original contract, any changes, and evidence of what each side gave or gave up.
Common misconceptions
“Any written promise is binding.” It usually needs consideration.
“A small change needs no paperwork.” Changes can create enforceability disputes.