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