What's the meaning of the word "that" in sentences like "He's not that smart"? How much does it change the sentence meaning and how is different from "He's not so smart"? Thank you.
"He's not that smart" and "He's not so smart" (and "he's not too smart") all mean "He's not very smart."
Theoretically, they could be contradicting a previous statement, such as "He has an IQ of 180," "No, he's not that smart!" ... however, out of such a context, it's often used to mean "he's not very smart."