What does the term central idea refer to within a primary source document?

it works as a summation  point of the source

Explanation:

In simple words, The fundamental idea or the central idea refers to  the core argument the author makes (also called the primary idea or the primary concept). In other phrases, what all the essay is all about is a core concept. You may conceive of a key concept as a sentence fragment: one phrase that says what the essay is about. There may be more for one core concept for a paragraph.

for why it was for independence

d.summerains, the last one