pseudo state in css code example

Example 1: css psedo class

button:hover {
  color: blue;
}

A pseudo-class is a word such as :hover that makes its changes
to the selected element by adding it along side the selector 
before the curly braces in CSS instead of inside the curly 
braces.

You can search for a list of pseudo-class online.

Example 2: What is a pseudo selector?

A pseudo-class is a selector that selects elements that are in a specific state, e.g. they are the first element of their type, or they are being hovered over by the mouse pointer