If you've seen HDR suddenly appear on your phone while you're taking a picture, you have probably done what most people do, ignored it. And you can, and really mostly should. If you're curious about it, it stands for High Dynamic Range, if anyone asks you, but that I doubt they will.
What it's really telling you, in simple terms, is that you are to take a terrible photograph. And it's because you are trying to take a picture that has some really dark areas and some really light areas. The dark areas will get way too dark, and the light areas will wash out. So, if you just want to keep it simple, when you see HDR on your camera, move until it goes away, or take a picture elsewhere.
The reason that what looked like a great picture turned out to be such a lousy photograph has to do with how your eyes work. Your eyes have built-in High Dynamic Range. That is, when you look out of the door, like in the photo I took above, the dark areas don't get so dark that you can't see any detail, and the light areas don't get washed out. Whether you give credit to evolution or design, you gotta admit that human eyes are pretty darned amazing. Of course, it's not just your eyes, it's that wonderful brain of yours.
So that's HDR. If you're human, and have healthy eyes and a healthy brain, you see that way all of the time. Cameras try to, and if you're curious about HDR, you can Google more stuff about it in Photoshop, cameras, etc., but be prepared for a lot of technical mumbo-jumbo. For me, I just look to see if HDR appears when I'm about to take a picture, and then I move, or don't take the picture at all.