The Compound node makes it possible to combine the contents of several frames into a single frame. The value you enter in the Compound Over field determines exactly how the frames are combined. This is similar to Average, but the resulting clip is shorter, according to the original length/value in the Compound Over field. See Creating Compound Images.
The First Processed Frame field sets the value at which output is processed from the node. Frames earlier than the entered value are included in the output but display the first processed frame.