It's a samsung app used to connect samsung devices together to get notifications etc. I've only played with it briefly the other day connecting my s9 to my s5e so I'm far from an expert. It worked though and was easy to set up. It has to be on both tablet and phone (on galaxy store if not pre-installed)and start the apps at the same time. I'm pretty sure there was an option for the watch too, but not sure if there is a specific app needed for the watch as well. I also have a galaxy watch but didn't try it with samsung flow.
I've since disabled it though as I found it too annoying to get the notification on my phone, watch and tablet.