Everyone has something that they absolutely love. That they call their "thing." On the spectrum, this is usually taken to an extreme. These are what are known as special interests.
When you like something so much that it's one of your main interests, leaving the many other possible hobbies and pastimes to the wayside, it may be considered a special interest. While most of us have things we really like, those of us on the spectrum may have an affinity for, say, computers at the exclusion of everything else. It is possible to have multiple special interests.
Mine are video games, the Android operating system, collecting charms and new songs sampling older ones. I've been drawn to video games for as long as I can remember. I'm a die-hard Sonic fan, to the point that was my nickname for several years, I have Sonic collectibles littering my room, and you can find me playing it several times a week. I'm always flashing my phone and testing the latest ROM, and people often look puzzled when I explain the different versions of Android based on the names of sweets. I'll hear a new song and can almost always come up with at least one older song I recognize in the background. I love the way charms sparkle and am especially attracted to those with moving parts.
The main thing is, when we on the spectrum like something, we really like it! We're very passionate about our interests and take them and run with them. I know that everyone will know I love Sonic just by talking to me for years to come :)