tells ffmpeg which streams to take from the input files and send as input to the concat filter. In this case, video stream 0 [0:v:0] and audio stream 0 [0:a:0] from input 0 (input1.mp4 in this example), and video stream 0 [1:v:0] and audio stream 0 [1:v:0] from input 1 (input2.webm), etc.