OLED screens produce true black displays by turning off the corresponding pixels. This means that in Dark Mode, any black areas of the screen will be shut off completely, saving battery power.
This doesn't mean you won't save any battery life with an LCD screen — like on the iPhone 11, iPhone XR, or iPhone 8 — just that the difference might not be as notable.
Source: Business Insider
To turn on dark mode on your iPhone, open up Settings, then go to Display & Brightness. You'll see light and dark mode, plus the option to change the mode depending on the time of day.
Watch the full video from PhoneBuff, which does a thorough battery life test, right here.