The purge option is an easy way to free up space on your account. It goes through the "data" directory where DiaGrid stores your simulation results, and discards all of the results that have built up since you started using DiaGrid, or since your last purge. You might not have known that simulation results were being kept on your account. In that case, you won't miss them, and "purge" is an easy way of cleaning up.
The default choice is to purge minimally, which means that the purge operation will delete the oldest simulation results first, and continue deleting newer and newer results, stopping as soon as you are under quota.
Other choices are older than 1 day or older than 7 days or older than 30 days, where you explicitly set the time range of simulation results that you want to purge. You can also choose to purge all, which means that all of your simulation results will be deleted.
If you want to keep some of your simulation results, you should choose the manually browse option and follow the instructions to access your file system via WebDAV. Once you have connected that way, you can move select simulation results from the "data" directory to another location where they won't be affected by the automatic purge. You could also browse through other directories and free up space manually.
Note that you might purge and still be over quota if you haven't done much simulation. In that case, you'll need to browse manually and look for other files that can be deleted. Or, you can contact the DiaGrid staff and beg for more storage space!