What EXACTLY does "contextualize" mean when referring to a historic document?

Answer 1

Basically what it means it that historic documents must be interpreted with reference to the "context" of the time in which they were written - the ideas, society, customs,
world-view that was predominant at that time.

Example: It's inappropriate to berate the Founding Fathers of the USA for allowing slavery and for not giving equal rights to women when, at the time the Constitution was written, slavery was still a widely accepted practice and women were not considered to be the equals of men.

Answer 2

To consider the document with in the context of the society that created it. This not only means to study the culture and language of the document, but also to look at how the document came to be. Who wrote it, why, and what bias might the author have had?