A breach that is material excuses the other party from further performance. One that is not gives a claim for damages but no right to stop. The distinction decides who ends up the breaching party.
Factors courts weigh include the extent to which the injured party is deprived of the benefit reasonably expected; whether that party can be adequately compensated in damages; the extent of forfeiture the breaching party would suffer; the likelihood of cure; and whether the breaching party acted in good faith.
Time of the essence. Late performance is ordinarily not material unless the contract makes time of the essence, or the circumstances do. Boilerplate saying so is given real weight.
Substantial performance. A party who has substantially performed may generally recover the contract price less the cost of completing or correcting — the doctrine exists so that a trivial defect does not forfeit an entire construction contract.
Cure periods. Most commercial contracts require written notice and a stated opportunity to cure before termination. Skipping that step is a common and serious error: a party with a genuinely good termination right loses it by terminating in the wrong manner.
The practical sequence. Document the breach, give the notice the contract requires, keep performing during the cure period unless the contract says otherwise, and get advice before declaring termination. Reversing a wrongful termination is far harder than delaying a rightful one.