Hell as depicted in Chainsaw Man is the location from which devils - manifestation of mankind's fears - originate, and to which they return after they die on Earth.
The embodiment of the fear of Hell - the Hell Devil (Chainsaw Man) - can transport individuals to hell from Earth and vice versa.
One portion of Hell appears to be a grassy, flowery field with hills, with countless doors parallel in the sky. It is unknown whether there is more to Hell beyond this area that has not been seen.