in accord with the most fashionable ideas or style; "wears only the latest style"; "the last thing in swimwear"; "knows the newest dances"; "cutting-edge technology"; "a with-it boutique"
fashionable and modern in the way that you dress, think etc = trendy with it with
If someone is not with it, they do not feel alert and therefore fail to understand things. She wasn't really with it. She hadn't taken in the practical consequences
If you say that someone is with it, you mean that they are fashionable or know about new things, especially in culture