This is a weekly process that creates a database used to respond to the "whatis" command. Whatis displays a brief description, taken from the man pages, for a command. For example:
$ whatis cron
cron (8) - daemon to execute scheduled commands (ISC Cron V4.1)
The man pages are stored in a compressed format, so makewhatis needs to expand them to get the description (which it does with zcat). Some packages don't follow that man formatting properly, or don't provide gzip'd man pages. When makewhatis does its processing every week, it will generate a few errors for these files.
If you find these occassional messages annoying, you can edit "/etc/cron.weekly/00-makewhatis.cron" and change:
makewhatis -w &>/dev/null
This will suppress any errors. If you never use the whatis command, you can simply erase the "/etc/cron.weekly/00-makewhatis.cron" file.