[linux-audio-user] new to linux audio

Hi everyone, I'm quite new to linux (and linux-audio) and have some basic questions regarding the audio tools on linux. I'm running Mandrake 9.0 with a patched 2.4.20 kernel and finally managed to install most of the applications I need (thanks to Thac's RPMs !!), mainly alsa, jack, ecasound, Muse, rosegarden, some soft-synth, did not try ardour yet ;-). I think I still need to enable capabilities in the kernel for jack, but I try to do things one by one. When reading articles on low-latency linux, xruns are often mentionned but I can not figure what it exactly means, could someone explain me that -surely- simple thing ? The other question is about jack. Does Jack enable synchronisation between HD recorder (ecasound, ardour) and midi sequencer (Muse for example) ? I saw something about the transport feature of jack, but does not know exactly what it does. Many Thank, Christophe