What sufficient means
A sufficient technique is a way of meeting a success criterion that W3C has vetted: do this correctly, in the situation it describes, and you satisfy the rule. Adding alt text to an informative image (H37) is sufficient for rule 1.1.1: for that image, nothing more is required.
Some rules have one sufficient route; others have several, organized by situation. Rule 1.1.1 alone lists different routes for informative images, controls, decoration, and CAPTCHAs, because sufficiency always depends on which situation your content is actually in.
The nuance everyone misses
WCAG requires you to meet its rules, not to use specific techniques. The techniques are informative, not normative: you can pass a rule with an approach W3C never documented, as long as the outcome genuinely satisfies the criterion for users.
In practice, though, the sufficient list is where every fix plan should start. These routes are documented, testable, and defensible. If a dispute ever asks "did you fix it properly?", pointing at a correctly-applied sufficient technique is the strongest possible answer.
In audits and fixes
Every finding in a WCAGrules report links the sufficient technique that resolves it. That is what turns an audit from a list of problems into a work plan: your developer opens the finding, follows the linked fix, and the retest confirms the rule now passes.
When we verify fixes, we check the outcome against the rule, but the fastest path to a passing retest is almost always the sufficient technique we cited.