KDirStat was a graphical disk usage utility for KDE 3, very much like the Unix "du" command. In addition to that, it came with some cleanup facilities to reclaim disk space.
But it's history now.
Use the newer QDirStat instead:
https://github.com/shundhammer/qdirstat
This repository here exists only for historical reasons. This old KDE 3 based KDirStat is no longer maintained.
(More screenshots at the old KDirStat home page)
http://kdirstat.sourceforge.net/
QDirStat is independent of any KDE libs or KDE environment; it only needs the Qt runtime libraries. It runs on any graphical Linux/Unix/BSD desktop. QDirStat is written and maintained by the original KDirStat author.
https://github.com/shundhammer/qdirstat
K4DirStat is a port of the old KDE 3 -based KDirStat to KDE 4 / Qt 4 and later KDE 5 / Qt 5. QDirStat is independent of that; it is based on the old KDirStat directly.