in theory*, studio monitors have a flatter frequency response, where as most other speakers have exaggerated bass and treble with less midrange. While that may be nice to listen to existing music on, it is not as easy to mix on because a whole bunch of audio information is difficult to hear - some might say the most important part of the audio - the midrange. The theory is that if you can get a good sounding mix on 'studio monitors' it will translate well to a wide variety of speakers, from ipod earbuds, to car stereos to computer speakers, etc..
*many will argue (and rightfully so, I'm sure) that high-end hi-fi speakers make better monitors than low-end studio monitors. Either, though, should be WAY better than your computer speakers.