Overpronation is widely understood to be a problem in which the foot is pronated too much. Pronation is the motion that is associated with an ankle joint tilting inwards and the arch of the foot collapsing. This is widely understood to increase the risk for injury from pronation in runners. Foot orthotics and motion control running shoes are widely used to treat this, but it is not that clear cut. There are many different causes of overpronation, so there can not be just one treatment for it. Also, there are a lot of people who overpronate who never get problems, so not all cases of it need to actually be treated. These are also a lot of strong and ill-informed opinions on the topic online about this, that can easily mislead runners into a poor understanding of the topic.
If you actually have “overpronation”, then find someone who actually knows something more than a superficial understanding of it; get the actual reason for it determined; and if it needs treating, make sure the treatment is directed at the cause. Most importantly, ignore so much nonsense that gets written online about it and read about the myths and hype.