Square brackets are a syntactic device used within quotations to clarify the speaker's meaning and modify his or her words without altering his or her meaning in any way. This is easiest to explain through example. Imagine that the following exchange took place.
EDB: Me like jclast. Me find him nodes good. And funny.
Halspal: Truly, he is a noder of the highest caliber.
Now, let us assume that you wish to quote Halspal in your biographical node. Without the square brackets, you would need to write something like this.
It has been said about jclast that "he is a noder of the highest caliber."
Although the preceding sentence is grammatically correct, it sounds awkward, and I don't like it much. With the square brackets at your disposal, you could write this.
Is has been said that "[jclast] is a noder of the highest caliber."
The square brackets are a tool that belong in every writer's repertoire. With them, you are free to write as you see fit, and quotations can be made to accommodate the author instead of vice versa.
References:
Mrs. Gunn's 12th grade Advanced English class