1Why and how
MITRE ATT&CK is a public knowledge base of adversary behaviour observed in real attacks. Tactics are the attacker's goals, such as Initial Access, Persistence, Lateral Movement or Command and Control. Techniques are how they achieve them, each with an ID like T1566.
Sub-techniques add a suffix: T1566.002 is Spearphishing Link, under Phishing. Using IDs means two teams describing the same attack mean exactly the same thing.
2Use it for coverage
Map your detections to techniques, and the gaps become visible: the techniques nothing would catch. The ATT&CK coverage page on this site does this for the incident library.
