A DPF - Diesel particulate filter is a filter that works in the exhaust system of later model diesel vehicles by capturing and storing soot in order to reduce emissions. When the DPF becomes blocked to a point where it can no longer regenerate on its own it will usually go into limp mode and throw a check engine light. There is a way you can do a forced regeneration yourself with a scan tool!
If you are new to this and would like a better understanding on how an OBD (on board diagnostics) scan tool actually works please take a read!