Like Peertube, videos shared on Pixelfed are now (optionally) WebP2P enabled, meaning that all users currently watching the video contribute to sharing the video by sending small parts of it to other users via a P2P connection similar to Bittorrent. This helps the server a lot as it only has to send the parts no other viewer can send. This is important when videos are very popular and watched by many people the same time, which would normally overload the server (unless you are Google and have datacenters for Youtube all over the world).