The way that it works is that many amateurs will run a client that will monitor received traffic for callsigns (the pattern 'de callsign callsign') and, when seen, will report this fact. This is of interest to the amateur who transmitted adn they will be able to see where their signal was received. The pattern chosen is typically part of a standard CQ call. The duplicate check is to make sure that the callsign is not corrupted.
The way that this would be used is that an amateur would call CQ and could then (within a few minutes) see where his signal was received. This can be useful in determining propagation conditions or in adjusting antenna and/or radio parameters. It will also provide an archive of reception records that can be used for research purposes.
The Webpage is http://pskreporter.info
There is a map display of this information.
There also a page of statistics about the project.