I don’t know what the “a distant mirror” is, but a primary source is a first hand account of a time or event that provides (original) insight. For example a journal is a primary source, anything written or made at the time of the event. Whereas a secondary source is a future analysis or interpretation about the event, like the big history textbooks the schools hand out.
So was it written in the 14th century or was it written later on about the 14th century.
I hope this was understandable and helps :)