A bookmarklet is a bookmark whose URL is JavaScript code instead of a site. It might be, for example,

javascript:document.querySelector('video').playbackRate = Number(prompt("speed")) || 1; void(0)

// formatted version:
javascript:
document
  .querySelector('video')
  .playbackRate = 
  Number(prompt("speed")) || 1; 
void(0)

so that if you click the bookmark, it sets the speed of the video to whatever you want (e.g. 3.7).

You could also run this directly in the URL bar (in some cases -- I think desktop Chrome does that), or you can simply type alert() into the dev console (desktop Firefox prefers this for security reasons).

Is running my own arbitrary JS like this a thing on mobile? I'm on Android but I'm not sure if Brave disabled it -- I vaguely remember it working once, but it doesn't anymore. No luck on Firefox either. Maybe there's a workaround?