I have experience in trading but mostly in lower frequency quantitative trading. I've moved into HFT research and someone the other day mentioned markouts. I couldn't find anything online explaining what markouts are.
I've heard it more commonly called a "markup", but either way, it's the difference between a current price (e.g. a trade price) and some microprice (midpt, or some fancier fair-value estimate) at some time in the future, typically some number of seconds, or quote updates. Sometimes people refer to just this future micro-price as the markup. It's used at the target value for fitting high-frequency models.