Runners rash or chafing is irritation of the skin caused by friction which can be skin on skin or skin on clothing. It can be the difference between a pleasant run and a very painful one, if you know you know! If ignored this mild skin irritation can lead to excruciating blisters and raw skin.
Generally speaking chafing is more likely during longer distances in hot/humid or wet conditions. It can also be impacted upon by body type, ill fitting clothing, prominent clothing/underwear seams, waist bands or back packs that dig in or rub and how much you sweat. It is very individual. Common places include groin area, nipples and under arm area.
The good news is that there are measures you can take to both prevent and treat chafing. Wearing tight, breathable, correct fitting seamless clothing is a good start. As is applying a lubricant to the high risk areas prior to run or even during if you feel an issue arising. Fortunately there are products available to help you along the way.