Text reflow is when you zoom in/out, it'll fit all the text into the screen so you dont have to move left to right to see what you're trying to read. The AOSP browser doesn't do it either (by default), but at least there's an option to enable text reflow. Miren doesn't. That's a deal breaker for me right now.
You can reflow by double tapping on the text you want to reflow.
I personally am not a fan of the pinch-to-zoom automatic reflow that some browsers have. When I pinch to zoom in on a button to click it for example, I don't want the whole screen to reflow and for the position of the button I was going to click on to change.
I much prefer the reflow in Miren.