Explain what a pseudo class selector is

A psuedo class selector is not defined explicitly in the markup. They can be used to specifiy style when a certain action is taking place regarding an element.
For example to specify the color of a div when hovering over it you could write in css.
theDivsIdSelector:hover {
  color: blue;
}
The pseudo class is accessed by using a selector like id or class name in order to access the element then adding a colon and the name of the pseudo class. 
You can find a list of other pseudo classes available by searching online.