SFUs, or Selective Forwarding Units, receive media streams from multiple participants but only forward a subset of those streams to certain participants. For example, if you're in a Webinar of 20 people, instead of opening up direct connections to all 19 other participants (and having every other participant do so as well), you can open up a direct connection to the presenter and an SFU which will send you an aggregated, downsampled stream of all the other participants. It reduces direct connections across large calls and saves on processing power across all participants by aggregation.
