HLS stands for HTTP Live Streaming. It’s an adaptive bitrate streaming protocol developed by Apple. One of those sentences to casually drop at any party. Äh. Back on track: HLS allows you to specify a playlist with multiple video sources in different resolutions. Based on available bandwidth these video sources can be switched and allow adaptive playback.
This is an interesting journey where the engineering team behind Kitchen Stories wanted to switch away from the Vimeo player (160 kB), but still use Vimeo as a video host because they provide direct video links with a Pro plan. Instead, they are using the native <video> element, a library for handling HLS, and a wrapper element to give them a little bonus UX.
This video stuff is hard to keep up with! There is another new format called AV1 that is apparently a big deal as YouTube and Netflix are both embracing it. Andrey Sitnik wrote about it here:
That doesn't even mention HLS, but I suppose that's because HSL is a streaming protocol, which still needs to stream in some sort of format.