What is meant by a misfolded protein for prions

A prion is a microscopic, infectious protein particle similar to a virus that lacks DNA or RNA. It is an external agent which when enters our body causes many disease. 

A prion is also referred to as a misfolded protein (folding which is different or not normal) which has the tendency to pass this misfolded shape to normal variants of the same protein.

Hope this clears your doubt.
Regards.