CSS pseudo-class selectors are special selectors that allow you to target elements based on their state or position in the document. They are used to style elements based on their current state, such as when a user hovers over a link or when an element is in focus.
From Wikipedia
Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling of a document written in a markup language, such as HTML or XML. CSS is a cornerstone technology of the World Wide Web, alongside HTML and JavaScript.