You probably know that tapping on the screen while in the camera will focus on the area you're tapping on. However, it also matches the exposure to whatever you're tapping on as well.
To test this out, point your phone at a window where a lot of light is coming in. If you tap on the window, everything outside will become visible, but everything inside will get pretty dark. If you tap on something outside of the frame of the window, the light coming through the window will get blown out, while everything in the room is visible.
If you want a little more control over this, you can tap and drag your finger up or down after tapping. Dragging down will set the exposure lower, as you can see above, and dragging up will bring the exposure higher.
If you want to lock your exposure and focus to a specific object, hold your finger down on that object for a few seconds until you see "AE/AF LOCK" appear at the top of the screen. Now you can point the camera anywhere you want, and the focus and exposure settings from what you tapped on will be saved.