The animation shows the client pc machine on the left and the server machine on the right.  An osi model is beneath each system.  An open envelope starts at the client, closes and moves down through layer 7, the presentation layer of the model.  As the envelope passes through each of the layers it increases in size until it is at layer 1.  From layer 1 the envelope disappears and we see a series of zeros and ones along with an analogue wave from pass between the client and the server.  On reaching the server, the envelope reappears and starts to go up the layers of the server starting at layer 1 the physical layer.  As it goes up each layer the envelope gets smaller until it reaches the top layer where it becomes an envelope again.  The whole movement is reversed and repeated from the server to the client.