A filter graph is used in multimedia processing - for example, to capture video from a webcam.
To compress a multimedia stream a filter graph could have two inputs: Usually these are expressed as file sources.
The file sources would feed compression filters, the output of the compression filters would feed into a multiplexer that would combine the two inputs and produce a single output.
(An example of a multiplexer would be an MPEG transport stream creator.)
Pins or pads identify the connection points between vertices and edges.