Read the full article here.
These are for MacPorts runtime data. The sub folders are:
build
It used during the building of ports. It normally contains empty folders.
distfiles
The downloaded sources of each build port.
packages
Contains the binary archives of each installed port.
registry
Contains detailed information about each installed ports.
software
Contains the archive of installed software in compressed format.
sources
Contains the Macport sources in different subfolders in different format.
Now removing/uninstalling MacPorts can be a drastic step, and depending on the issue you are experiencing, you may not need to do so. Please ask on the macports-users mailing list first, if you're unsure.
Asking for answers to be accepted are frowned upon. They add unnecessary noise to the comment section. You should consider improving this answer, you mention a link, answers which contain a link should reference the material from the link and quote all relevant information. If there is no relevant information to quote (i.e. its a download link) then specify the fact, links which are not quoted really shouldn't be provided though. – Ramhound – 2016-08-06T09:27:00.263