I'm sure I've heard multiple variations on this expression, but I can't clearly recall any of them, and searching without the key word is impossible. Basically it's a phrase meaning you strongly agree with what was just said and want to emphasize that, possibly following up with more explanation of why that statement was so true.
Kittens are the best!
This with [blank] on. They're so fluffy, and their cute little meows melt my heart.