What is difference between KML (filtered) and KML (full) downloads within ForeFlight on the website?

Typically, Track Log data points are recorded every 1 second (1 Hz).  If you use a Stratus 2 to record Track Logs, the data is recorded every 1/5th of a second (5 Hz). 

The KML (Full) download is a file with all the Track Log data that was recorded for a particular flight.  This will be a 1 Hz recording if you used the 'REC' button on the Maps page.  If you used a Stratus 2 for the recording, it will be a 5 Hz recording.  

The KML (Filtered) download is Track Log data that is reduced to 1 Hz, which allows for much longer flights to be displayed within Google Earth.  Google Earth has a limit on the number of points within a KML file, so a 1 Hz file will be able to display a longer duration flight.  The KML (Filtered) option will not change files that were recorded at 1 Hz.  

Image 1:  Track Log KML file options within ForeFlight on the web.