A head-mounted display is a device that does require an image source, which means the image that is to be displayed, a device that really makes it possible for the image to be seen by the eyes, and that seeing process happens to optical viewer. It also consists of a coupling system, a visual coupling system, that automatically controls what exactly is being displayed. In other words the optical coupling device, a visual coupling device, tells to the sense to the display what image, whichever image to be displayed, and that the criterion for what has to be sent is user-determined.
Then the HMD itself, can have the property of being in monocular, which means you only see something in one eye, or you can see the same thing in both eyes, identical things meaning a repetition of what you saw in one eye into the other, so that really you see the same image twice, or you can have a much more realistic, where you really see two different images which are in a such way transformed that is one image is transformed in a certain way before it is mapped to the display of a second eye, so that these is now a binocular display.
These are similar to what we experience, we have two different images reaching our eyes from the reality, so binocular HMDs actually show two different images of the same scene to the two eyes.