Basically, as you drill down by clicking on subdirectories, it will show only the content used in that new subdirectory or its child subdirectories. You’re going through a forest, and searching for smaller and smaller patches until you see something out of place.
In the first filelight image I see “/usr” contains the bulk of used space, although “/home” is significant. Probably, if you drill down on “/usr”, then you can go to its largest subdirectory, and see what is there. If you get to a subdirectory which is large, but has only a few files, then you might have found the offending file…if not a single file, then there may be a lot of smaller files taking up your storage. Basically you need to drill down on the largest “slice of the pie” until you find something “optional” (something not part of the actual operating system) which might be unneeded.
Each subdirectory of a directory is a collection of files or other subdirectories. Just keep drilling down until you find something which shouldn’t be there, or something which could be removed (just make sure you remove it via the package tool if it is part of a package…most /home directory content is not a package, most /usr content is a package).