What is in /opt/local/var/db/dports

Hi, I just checked my system for places where I can get a few megabytes of diskspace back and found that /opt/local/var/db/dports contains quite a lot of files. Essentially all software packages I installed are there once again. Is it safe to delete them? Is there a general procedure to free as much space as possible while keeping the ports intact? Thanks in advance for any pointers, Markus