Debug. Explain "Object reference not set to an instance of an object"

If possible, please consider to better explain or clarify what, in my opinion, is the FuseTools “blue screen error” (in the sense of a cryptic error) ;-))

"Object reference not set to an instance of an object"

by adding some more informations such as what object or missing object generated the error or what reference was not set.

Thank you.