Unknown skin condition outdoor kitten

Hello!  Is this your cat?  You don't mention any history.  I wonder if this cat has had knotted fur over the flanks, perhaps caused by an inability to groom (arthritis?  Spinal pain?) And / or infection or parasitic involvement.   What happens next must depend on the situation; if it's your cat, a veterinary check-up and - if that fur is knotted- even a dematt / shave may be in order. Please note that I am basing my comments on a photograph; a vet would be able to examine the lesion properly to understand what is going on.