E-E-A-T stands for Experience, Expertise, Authoritativeness and Trustworthiness. Google uses the term in its guidance on helpful, reliable, people-first content to describe the qualities by which you can recognise good content: first-hand experience, real subject knowledge, a recognised reputation and information a reader can rely on.
Is E-E-A-T a ranking factor?
No, not as a single factor. Google writes that E-E-A-T itself is not a specific ranking factor. Its systems do use a mix of signals that help identify content with these qualities. Of the four parts, Google calls trust the most important; the other three contribute to it. A page does not need to show all four equally strongly.
Example
Say a heating engineer in Apeldoorn writes about replacing a central heating boiler. Photos from his own jobs and the mistakes he regularly comes across show experience. His certification and technical knowledge show expertise. A clear sender with an address and contact details makes the page trustworthy.
Common mistake
Treating E-E-A-T as a checklist, for example by adding an author box under every page while the text itself shows no experience at all. What counts is what the content demonstrates.
For practical ways to show this, read E-E-A-T: how do you show experience and expertise?. For topics about health or money, E-E-A-T carries more weight: see YMYL.