Flow Apps are based on Node-RED, a tool to write flow-based programs. Node-RED provides an editor to easily build a visual representation of the processes that form a program. If you're not familiar with the concept of Node-RED flows then please look it up here: https://nodered.org/docs/tutorials/first-flow
This visual representation is a flow that consists of nodes that are linked together. Each node has a specific purpose. It takes an Input and runs a function or adds some data and export the new values as an output. Flow-based programming allows you to quickly set up a new application and helps to better understand what each component of your program is doing.
To start building flows, click on the Open Flow Editor button in your Flow App window. This opens the editor. You can create a new project or import an existing one.