Dark mode inverts the UI on the phone from dark text on a white background to white text on a dark background. It prevents the user and any passersby from having their retinas melted by the blaring white background light during the night or in a dark room. It also helps extend battery life a little on phones with an AMOLED display. That’s because such panels create the color black by turning off pixels in the appropriate location. Pixels that are turned off don’t use the phone’s battery, so the more black on the screen, the less energy the phone is consuming.
If you haven’t received the update yet, check back in later during the week. Remember, Google says that it will be rolling out the update for the rest of the week.