It is the melted exterior of the meteor. Because meteorites are composed of different materials not all fusion crusts look the same. Once a meteorite is on the ground it starts weathering. This weathering causes an oxidation of the crust and it turns from black to brown in color. Depending upon the climate conditions where it falls this will take varying amounts of time.
Fusion crust is one of the more difficult things for a novice to get a good feeling for.
It really requires looking at a number of different meteorites. Many people picture the crust as being like the skin on an orange, but it is much more like the skin on an apple. Fusion crust is very thin, usually less than 1mm thick. It is generally the thickness of a heavy piece of paper. Out west it is many times confused with desert varnish which is a dark coating on rocks exposed to the sun and elements for a long time.