What does the "D" on the signal strength bar for a given GPS satellite represent?

In the really old days of GPS the positional errors were often in the 10s of meters. A system known as differential GPS was developed which used corrections from a known location to correct some of the error. Originally this was only available in limited locations near a ground station and with special receivers. Eventually the corrections were available from a few additional satellites and on most GPS receivers.

Still referred to as Differential GPS, the satellites for which corrected information was available had a small 'D' showing on the signal strength bar.

The writeup at the following link gives a much better description: http://www.gpsinformation.org/dale/dgps.htm