This is an interesting development that Valve are currently working on and will be supported in Steam on PC and Steam Deck. I think some other developers are also developing this technology.

Basically what it means is if you have more than 1 PC or compatible system (Steam Deck) on your local network if you go to download a game, a DLC or an update, if another system on your network has already downloaded it then the client will instead download it from the other local system, rather then needing to download it over the internet.

For those on very fast fibre connrctions this won't be of much use, but for those still on relatively slow connections this will make a big difference.