In Menu Mode you can access your ship's computer, set configuration options and get help. However, most of the time you will be using this mode to set exploration destinations and engage your warp engines. See the screenshot below for detailed descriptions.
The other main gameplay mode is Flight Mode, and you use the [ESC] key to switch between them.
This shows the Menu Mode. Top left is the Computer button, followed in turn by the Planet Navigation, System Navigation and Warp Navigation buttons. Below the Computer button is the Chat button, and the top right corner top-to-bottom are the Quit, Config and Help buttons.
The bar towards the bottom of the screen just lists the next quest objective.