Breach

Not every breach is the same. A "material breach" goes to the heart of the deal: it lets the non-breaching party walk away from the contract entirely and sue for damages. A "minor breach" doesn't excuse the other side from performing, but they can still recover damages for the specific failure.

The non-breaching party also has a duty to mitigate damages: to take reasonable steps to reduce the harm. If your tenant breaches a lease and moves out, you can't just sit on the empty unit and sue for the full year's rent: you have to try to find a new tenant.