I'm pretty sure that he means that you should read something in an effort to understand the author's thought process, so that you can formulate your own opinion. This is opposed to reading things as gospel and believing everything you read.
It also protects against overly-cynical literal interpretations—if you read something and think "no one would be stupid enough to write that," then—since you can understand that the author was clearly attempting to make a certain argument, whether or not they succeeded—you should assume they said what they were trying to say, instead of bailing out like a compiler. The Principle of Charity, in other words.
"I believe virtually anything I read, and I believe that makes me more of a selective human than someone who doesn't believe anything." - David St. Hubbins
