When the mouse is over it, it has the hover property. And when it is one we have visited, then it has the visited property.
Anchor is the selector.
The 4 options are technically called pseudo class selectors. This is just the same sort of code applied to three of the conditions of the anchor. In all cases we make the font bolder. We have used a different value for the font family in the 'hover' case. And we have used different colours.