This is one of Google’s most important quality metrics that aims to measure the usefulness and credibility of your site and its pages. Google wants to deliver the most useful possible content to users from sources it can trust and this metric analyses a wide range of signals to determine these factors.
The amount of expertise, authoritativeness, and trustworthiness (E-A-T) that a webpage/website has is very important. Main content quality and amount, website information, and website reputation all inform the E-A-T of a website.
Keep in mind that there are expert websites of all types, even gossip websites, fashion websites, humor websites, forums, and Q&A pages, etc. In fact, some types of information are found almost exclusively on forums and discussions, where a community of experts can provide valuable perspectives on specific topics.
Some topics require less formal expertise. Many people write extremely detailed, helpful reviews of products or restaurants. Many people share tips and life experiences on forums, blogs, etc. These ordinary people may be considered experts in topics where they have life experience. If it seems as if the person creating the content has the type and amount of life experience to make him or her an expert on the topic, Google will value this everyday expertise and not penalize the person/webpage/website for not having formal education or training in the field.