The 'ch' stands for checksum.
The PageRank files can be requested directly from the domain
www.google.com. Basically, the URLs for those files look like follows (without line breaks):
http://www.google.com/search?
client=navclient-auto&
ch=0123456789&
features=Rank&
q=info:
http://www.domain.com/
replace 0123456789 with the checksum you receive for your site, and replace
http://www.domain.com/ with your sites url you calculated the checksum for.