A lot of information is displayed on the HUD, which is also available from the other displays in the cockpit. However, it is helpful to have all the information bundled in one place so that you don't have to lose sight of the target.
The displays on the HUD are adjusted according to the situation. This means that when a weapon is activated, it looks different than when you are flying, and even if you transit flying, parts of the display change depending on altitude, speed, etc.
The picture below should only give you a rough overview of the different elements.
The HUD will be discussed in the Extension course.