CBS stands for component based servicing. CBS.log is a file which include logs about components when they get installed or uninstalled during updates.
If you want to check these files, they are located at %windir%\Logs\CBS\
You will see two files in CBS folder, one is CBS.log and the other is CBS.persist.log. CBS.persist.log is the older of these two and is generated when the CBS.log is around 35-40 MB. Persist log folder can easily be dumped.
You need administrator rights to open these log files.