Just like in the first game Flow Free you need to cover all of the grid space and join up every node but you now also need to navigate the bridge featured on each level. One of the lines you draw must pass through the bridge vertically, while another needs to go through it vertically before they both finish up at their destination points.
There are four criteria for succeeding at a level: you need to join together dots of the same color in one continuous line. They mustn't overlap at any point. You can't leave a single piece of the grid uncovered. Two of the lines must pass through the bridge that now features on each level.