Your camera likely doesn’t have this, unless you already know it is there. You may not know what it is for, even if your camera has this. I use this sometimes and at those times I could do without it, but only with considerable hassle.
No, nevermind the grownass man wearing short pants, or the human wearing sandals with socks, or the BLACK socks with short denim pants. PLEASE disregard the horrible physique and the horrible fit. No, this is more about the exposure. I wanted to take a picture of the Champion of All Fashion and I went point-and-shoot. Automatic exposure got me the top picture. The sky is properly exposed, the sun is somewhat controlled with just a big sunstar and a little ghosting. The intended subject is basically all black. I then pressed this magical button.
I had previously set this button to AE-L. This tells the camera, when I press the button, to adjust the exposure to whatever the camera sees at that moment. I pointed the camera at a dark part of the minivan, locked exposure, recomposed, and got off a quick snap with something approaching focus somewhere inside the minivan. Cut me some slack on the focus here I was trying to be quick/discreet.
The point is, when the computer chose how to expose the frame, it chose to not blind itself with the sun and nailed the exposure of the sky. I didn’t care about the sun or sky, I wanted the dark blob in the bottom of the frame exposed so I could see what was going on. The only additional thing I had to do was push this one magical button. The alternate setting for this button is to lock focus, but half-pressing the shutter release does this anyway, so I use that button for that function.
It’s in your manual. Look it up. It can save some time, sometimes.