It means either you're not uploading enough or the torrent's host that you're downloading isnt uploading enough.
Torrentz work by other users called "peers" uploading some of their transfer limit to the torrent so others can download it. The side effect is that people called "leechers" then come and start leeching off these uploads without uploading some of their transfer limit for themselves meaning some poor guy is going to be downloading at 10kbps when they could be downloading at 128kbps if everyone was uploading enough.
Alternatively, it may be that your torrent downloading program hasn't been configured to download more then 10kbps. If you just type in "how to increase download for (insert torrent program name here)" in google it will tell you what options to click to increase your downloading speed.