Definition
A feature that keeps the clock and notification icons showing on your phone's screen even after you lock it. AOD is short for "Always-On Display." Instead of lighting the whole screen, it dimly lights only the spots where the text and icons sit, and usually leaves the big background off. So you can still see information, and the battery drains only a little.
In a dark apartment building, only a few windows are lit
Look up at an apartment complex at night, and most of the windows are dark, with only a few lit. You can still make out the shape of the building at a glance. A few lit windows are enough.
A modern phone screen looks a lot like that building. A screen is made of millions of tiny dots called pixels. Each pixel is like one window.
Many screens today have pixels that each make their own light. So a pixel that looks black isn't giving off any light at all. It's just resting. A black background is like a dark window, and it uses almost no power.
This feature takes advantage of exactly that. It lights only the pixels where the clock digits and notification icons sit, and keeps the rest of the background off. That's why the battery barely drops even though the screen is on.
Three tricks for saving light
The first trick is to light fewer windows. An AOD screen usually leaves the background pure black and draws the text with thin white lines only. Some devices, like the iPhone, show the wallpaper turned way down and dim, but even then they cut the brightness and the number of times the screen is redrawn by a lot. That's why you see thin digits instead of a fancy wallpaper.
The second trick is to turn the light down. Even the pixels that are on use less power when their brightness is set low. That's also why an AOD screen in a dark room looks dim instead of dazzling.
The third trick is to redraw the screen less often. A normal screen keeps repainting the picture at a speed you can't see. But a clock only changes once a minute, so there's no reason to redraw that often.
Put the three tricks together and, though it varies by device and settings, the battery drains much less than when the screen is fully lit. You're lighting a few windows, very faintly, and only once in a while.
To be more precise
Leaving it on for a long time creates one problem. If the pixels in the same spot keep glowing, that part wears out first and leaves a faint mark. It's like a single window that stays lit for years: the color of that window frame fades first.
This kind of leftover mark is called burn-in. That's why phones don't pin the clock and icons in one place. Every few minutes they redraw them shifted a little, so no single pixel gets overworked.
Also, not every screen can use this feature thriftily. If one lamp behind the screen lights the whole thing, there's no way to pull off the trick of lighting just a few windows. This feature saves battery only on a screen where a pixel that's off is truly off.
Common misconceptions
The screen stays on all the time, so using AOD drains the battery fast.
It dimly lights only the pixels where the text sits and keeps the rest off, so it uses very little power. It isn't zero, though, so turning it off helps a little when your battery is running low.
AOD just turns down the lock screen's brightness and leaves it on.
It's not only about brightness. It lights fewer pixels and also lowers how often the screen is redrawn, cutting power use in several ways at once.
Where you meet it
A quick glance at your phone on the desk tells you what time it is and whether a new message has arrived.
In class, you can spot which app has news just by seeing its notification icon, without turning the screen on.
In one sentence
A feature that dimly lights only the pixels where the clock and icons sit, instead of the whole screen, so it keeps showing information while using only a little battery.