UBB.threads PHP Forum Software Community Forums UBB.classic UBB.classic 6 Support 'core' files?

Can someone tell me what the "core.xxxx" files are for in the cgi-directory? I'm running way low on space on my ISP and these files are taking up almost 200MBs. I opened one of them, but can't make much sense of it. There seem to be many more than the number of forums I have.

Can I delete these files, or are they critical?

Thanks in advance!