One of the key things that gets pounded into my head as a photographer, and deserves to be at the forefront of any composition, is the necessity of achieving proper focus. This is harder than it sounds, and focus can often be off when you look closely at a picture you otherwise think is well composed.
And the reason focus is important is it’s one of the few things you can’t fix with an editor.
Part (but not all) of the problem is in the fact that modern cameras (and phones) are trying to think for you. Much of the time, these devices do the right thing – but certainly not always. The upshot is that once you acquire confidence to operate your camera properly, you will want to be in charge of this key process.
One of the techniques to achieve focus is documented in part here. Back-button focus (for DSLRs) simply means that pressing the shutter button halfway down and holding it is not always the best way to dynamically focus. Being in control of when your camera tries to autofocus and when to leave it locked in on your subject is the key benefit with this technique. Check out this great article by James Brandon.