CSS is an acronym for Cascading Style Sheets which are nothing more than a text document or two created by web developers to explicitly detail how a users web browser should present the content. HTML is about structure, CSS is about presentation. That is the easiest way I can explain it; hope I did not lose you.
Here is a tutorial on how to do this effect for you own local meet using CSS3.
Pretty cool stuff. These 'virtual graphics' for sports always intrigue me because there is practically zero time to edit it into the shot. Everything has to be done in real-time. Graphics like this also appear in many other high-profile sporting events like the Super Bowl and the World Series, where the ball is tracked three-dimensionally live.
Now, my school just happened to be hosting a swimming meet between several other schools. We were going to broadcast and record it anyway, so I thought I might as well try to recreate the graphics. In the end, what I came up with was pretty much visually the same as the one you see in the Olympics.