A failure by one party to perform an obligation the contract requires, whether or not the other party treats it as such at the time.
A breach is a question of what happened against what was promised, so it does not stop being a breach because both parties carried on cheerfully and nobody raised it. Nor does it automatically entitle the injured party to anything: entitlement usually depends on following a process, often within a deadline, so the remedy can be lost while the breach remains real. The practical consequence for anyone running delivery is that the informal path and the formal record are not alternatives. Recording that an event happened, and when it was known, is what keeps the informal resolution from becoming the only one available.
Where this comes up
A delivery manager does not need to know contract law. They need to notice the moment a delivery problem becomes a contractual one, because after that moment informal handling starts costing money.
A make or buy analysis compares the whole life cost of building something against buying it, and finds the point where the cheaper option changes. Capability, control and risk usually decide it.