This is when you are feeding power onto the grid. It happens during the day, and the energy that you don't consume is used by your neighbors. The utility company credits you that energy, and as you consume power at night, you use the credits that you accumulated during the daytime.
This is a digital meter.
If you notice, underneath the "4" there are two squares, and an arrow pointing left. That signifies backfeeding. If the arrow was to the right of the squares, and pointing right, then you would be consuming power from the grid.
The utility companies switch your analog meter out to a digital "Net" meter (see the red sticker on the bottom right?) when your solar power system is installed. However, occasionally, you get the opportunity breifly to see what the system does to an original analog meter.
This is an analog meter on an installation that has been recently installed. At the beginning, the meter is spinning backwards. When the solar power is disconnected, the meter stops its backwards direction, then slowly begins spinning forward again.