The best way I can describe it at the 1000 meter level is that basically, concerns give you a way to associate independent classes and/or code to a model that should not be inside the model itself. You will find a lot of "opinions" on how to use then and how not to use them.
There is a pretty good article on Medium about the correct and incorrect way to use concerns by Carles Climent.
medium.com/@carlescliment/about-rails-concerns-a6…
Hope this helps.
Mike