Flowing is the system of debate note-taking. You need to get down what arguments your opponents are making throughout the round, how those arguments change and adapt to the arguments you've made, and vice versa. In that, you will start on one side of the paper with the cases, then move to a more rightward column with the rebuttal, then the summaries, then the final focuses. There's a nice video here from Beyond Resolved that you can watch.
Most of the time, each side's case will be on different sheets of paper. Some do it on one piece of paper, with the pro side on the front and the con side on the back, or others use two separate pieces of paper. I personally use the latter, since I find for especially long cases I might have to go back on the back.
The picture on the side is an example of one of the sides of the flow from a debate I had. Specifically, this is our opponent's case (in red on the left), my rebuttal (in blue), their frontlining in rebuttal (in red again), my partner's backlining and extensions (in darker blue), and their re-frontlines and extensions (in orange).
As the debate progresses, some arguments made earlier on aren't brought up again. That's to be expected, given that those second-half speeches are shorter than the constructives and rebuttals. Flowing well allows you to track which arguments you're still talking about, and what arguments from your opponent you should be responding to.