The final_clip is a clip that plays the clips 1, 2, and 3 one after the other. Note that the clips do not need to be the same size. If they arent’s they will all appear centered in a clip large enough to contain the biggest of them, with optionnally a color of your choosing to fill the borders. You have many other options there (see the doc of the function). You can for instance play a transition clip between the clips with the option transition=my_clip.