You can find the details of what you're looking for in the database documentation. There's a link to it at the top of the page when you open the report editor.
WHERE
  tblDiskdrives.Caption Like '%c%'                           -- drive caption contains "C" (caption is C:, D:, etc.)
  AND Ceiling(tblDiskdrives.Freespace / 1024 / 1024) < 1024  -- free space < 1024 MB (aka 1 GB)
  AND Ceiling(tblDiskdrives.Size / 1024 / 1024) <> 0         -- drive is not empty
  AND tblAssetCustom.State = 1                               -- asset is active
  AND tblDiskdrives.DriveType = 3                            -- local disk drive