What is an xrun and how can I stop getting them?

If you get an xrun, it basically means that your systems CPU wasn't able to keep up with the information it was being fed from your soundcard. There are many factors at play.

More information about the causes of xruns

If you choose an audio orientated Linux distro, you should have less problems with xruns as these distros take into account many of the factors which can cause these problems. Note though that there are many factors at play, including your computer and audio interface. These factors can't be taken into account but an audio orientated Linux distro can help make a big difference.