Your windshield serves as a giant screen, and a projector embedded in the dashboard sends a transparent image onto it. The image leaves the projector and bounces off a series of mirrors, magnifying and flipping the image so that the data comes out right-side up and legible. Press a toggle switch by the dashboard and you can adjust where the display is on your windshield something accomplished with a rotating mirror.
Some car companies project their heads-up data onto a small plastic window that flips up, known as a combiner. This creates a uniform image no matter how the windshield's shaped. It also reduces the number of mirrors needed to send the image onto the windshield clearly. And a lot of aftermarket displays use such a combiner so you can install it in nearly any vehicle you please.
These displays give you information like current speed, revs, the media playing in your car, navigation, and other critical information that you will require while driving. The aim is to keep your eyes on the road at all times and not have to look anywhere else in the cabin while driving, thereby giving you more time to focus on the road. These systems plug into your cars on-board diagnostics and supply information to you, real time. For cars that are a little older, there is even the option of retro-fitting a head-up display via an aftermarket option. This, however, needs to be done in the right manner so that it is installed seamlessly by a professional.