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.