what are sudo selectors in css code example

Example 1: pseudo class css

.element:nth-child(1)
.element:hover
.element:visited

Example 2: 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.