As you run Linux, one of the many things that happens are logs being written, files being cached, search and file history being saved etc.
About once a month, we like to clean house, free up disk space and generally make things neat and tidy again.
You will actually find two menu items in your System Tools menu once installed: BleachBit, and BleachBit (as root)
Once you open BleachBit, you will be presented with a long list of check boxes for all the different cache and log files you can expunge from your drive. You can also then overwrite all the free space on your drive, making a forensic search almost impossible.
Word of caution though, be careful what boxes you check! Losing all your Firefox passwords, or email configuration can be a real pain to put right!