mp4box:tldr:871b0

The command "mp4box -add ${input1-mp4}#audio -add ${input2-mp4}#video ${output-mp4}" is using the mp4box utility to add audio and video tracks from two different input MP4 files and create a new output MP4 file.

Here's a breakdown of the command:

• "mp4box" is the name of the command-line program used for working with MP4 files.
• "-add" is an option that specifies that you want to add tracks to the output file.
• "${input1-mp4}" is a placeholder for the first input MP4 file. You need to replace it with the actual file path/name.
• "#audio" is used to specify that you want to add the audio track from the first input file.
• "${input2-mp4}" is a placeholder for the second input MP4 file. Similar to the first input, you need to replace it with the actual file path/name.
• "#video" is used to specify that you want to add the video track from the second input file.
• "${output-mp4}" is a placeholder for the output MP4 file. You also need to replace it with the desired file path/name.

Overall, this command is combining the audio track from the first input MP4 file with the video track from the second input MP4 file to create a new output MP4 file.