Extended definition
The visible text of a link. "Click here" is weak anchor text; "complete guide to Core Web Vitals" is good anchor text.
Context and application
For SEO, anchor text is a strong signal — Google uses the anchor text as a description of the destination page. But over-optimization (all external backlinks with exact match anchor “SEO audit”) = Penguin penalty. Natural diversity: ~30% exact match, ~30% partial match, ~30% brand/URL, ~10% generic (“click here”, “this page”). For internal linking (your total control), you can be more descriptively consistent — it’s not penalized.
In practice
descriptive anchor text helps Google understand what the destination page is about.