Locking your iPhone's exposure and focus is pretty simple. Tap the screen where you want to set focus and exposure, but instead of releasing your finger hold it down for a couple of seconds. A larger square will appear where your finger is along with a yellow "AE/AF LOCK" sign. That's it, we're locked in.
Of course, you're probably asking "Why would I do this?". Great question. What we did was ensure that no matter how the scene changes or what moves in or out, the focus and exposure are set. Basically turning off auto-focus. So when a subject or that adorable dog runs into the scene, the camera will not try to focus in on it. Allowing a more creative film and you, as the director, deciding how it's shot.