The FilesForm lets you select the AVI you want to join and the order in which they are joined. Here you can also select the order of the AVIs. The TimelineForm lets you select what segment of an individual AVI you want to use. The top track bar lets you navigate in the current avi and select the range of frames. Upper triangles and the blue bar shows start and stop of this range.
Lower ones show the position of cut frames.
The bottom track bar lets navigate in the final avi. The uper triangles show were the constituent avis start. The lower triangles show were the final avi is going to be cut. You can play all frames, not only the selected range. Actually not the final, but the selected frames from all files.