Hi,
lsof (list open files) is a command used to list open files, which with the concept of the virtual file system can in fact be network sockets, block special files, etc. (everything in linux/unix is a file). For example :
lsof -i TCP:23 (sockets using TCP port 23)
lsof /dev/hda5 (open files on /dev/hda5)
So, I would presume that you lsof.log is some kind of log file containing output from lsof. Exactly whats in it or where its created is difficult to say without more info...
Hope this helps