What do the brackets represent?

In quoted material a bracket around text indicates that it has been changes from the original quote. e.g.

"I was with him on the night in question."

"I was with [Michael] on the night in question."

Indicates that the Michael was not the Word used in the original quote, but gives a reader information that they were unable to infer from the Word him, but is still the accurate interpretation of the actual original quote.