What it is. MITRE ATT&CK is a freely available knowledge base of adversary tactics and techniques based on real-world observations. Tactics describe the attacker's goal at a stage, such as Initial Access or Exfiltration, and techniques describe how they achieve it. Separate matrices cover Enterprise, Mobile and ICS environments.
How to use it. Defenders use ATT&CK as a common language to map threat intelligence, assess detection coverage, plan red and purple team exercises, and prioritise controls against the techniques most used by relevant threat groups. Mapping logs and detections to techniques shows where visibility is missing.
Common pitfalls. Treating the matrix as a checklist to colour in leads to shallow coverage claims; one detection rarely covers every variant of a technique. Prioritise by relevance to your threats, test detections in practice and remember that ATT&CK describes behaviour, not severity or likelihood.