“扩展JPanel”是什么意思?

I have a class called UserInterface. In the code public class UserInterface extends JPanel, does "extends JPanel" mean that there is a JPanel within the JFrame (the JFrame is the application window) or does it mean something else?

另外,如果这意味着JFrame中存在一个JPanel,那么我如何访问JPanel(例如,设置其背景颜色)?