While not a standard industry term, it likely refers to one of the following:

The phrase "" appears to be a specific search query or a system-generated instruction used within media downloading tools or file management applications.

: " mp4 " indicates the MPEG-4 Part 14 container format, which is the most widely supported video file type for mobile and web playback.

Essentially, this instruction tells a program to "download the 23rd item in a list using the smallest possible MP4 file size."