What are these four letters
Experience – the author's personal experience, Expertise – their qualifications, Authoritativeness – recognition by others, Trustworthiness – the reliability of the website itself. The first E was added later: Google started distinguishing "a person did this" from "a person read about this".
What this means in practice
An article without an author and no traces of real experience loses to the same article signed by a specialist with examples from work. For a service website, this means a team page, real case studies, and easily verifiable contacts.
Who this is critical for
This is most strictly applied to topics where a mistake is costly: medicine, finance, law. But the principle works everywhere, and with the advent of answers from AI models it has become more important: they cite those who give reasons to be trusted.