Every flex application is composed of so many visual components. Every component resides in any container. Flex provides several containers to lay child components, each with different way of positioning. For example, some containers may position the child components horizontally, some vertically and some put the child components in tabular format etc. Container can contain visual components and other containers also inside it. Application is the top container for any flex application.
You can learn about flex containers going through the following topic: