Born w/flat feet & fallen arches. Slight pronation on sprained ankle than other foot. What does traumatic tendonitis secondary to pronation mean?

Basically. Pronation caused inflammation in one of your tendons. Usually it's the posterior tibial tendon.
Hurt tendon. it means that the tendons in your feet are chronically stressed and swollen due to the anatomic abnormality of excess long term pronation. Consider orthotics.