This is a big one!
Out of all the lessons this may be the most important. When taking a photo on a DSLR (real camera) You shoot in RAW, this is a file format that essentially is just a ton of raw data on each image, this allows you to edit and manipulate that photo on the computer to produce the final result. You want to do the same thing with your iphone photos. You take a picture, but from there you want it to look its best. This is where editing comes into play. We can use VSCO as the example app, once you take a photo there is a world of options for editing, including exposure, contrast, sharpening, fade, shadows/highlights, tone, and more. The best way to figure out what these actions do is to play with them and see. I generally like to bring the exposure of my photo up a little bit, add a little sharpening, and apply a filter that makes the photo pop. Even Instagram itself has nice editing features, although most of their filters should be avoided as they make photos look overprocessed and cheap.