Breaking that down, the 'a' at the beginning selects all <a> elements on the page, then applies a normal font size, makes the text blue, and gets rid of the underline. The next block that starts with a:hover selects all <a>s again, but then filters them to only apply the styles to those that match the pseudoclass ':hover', i.e. those that are in the state 'being hovered over'. Here we up the font size (1.2em, i.e. 120% of normal) and make the text red. Notice that we don't need to redefine the text-decoration statement, as we've already applied that for all <a>s in the block above. We could though of course add a
text-decoration:underline; rule on :hover so that when you hover over a link it gains an underline.