It means like, if you work together with a person or many people. You can do the impossible things. Like for example, building the most tallest house or just a simple house. If you don't have unity and cooperation with each other. You might even finish the house after many centuries. So, there is a real need to work together. You only need unity and cooperation with each other so that, you might even finish the house only after a few years or even only a few months.