What does the position of the tuning marker mean?
When you're trying to tune a string you play it and the Standard Guitar Tuner app will show you a string marker. It looks like almost like a string, but is red, yellow or green. The position shows you if your string is above or below the string in the background. Say you're trying to tune the A-string (the second one from the left) and it's too loose, which means that the tone it will play is too low, then the tuning marker will be to the left of the A-string of the guitar in the background, which should tell you that you need to tighten the string. That all sounded a lot more complicated than it is. You'd probably do better if you just play a string and adjust it until it's on the right string on the screen. You'll get the hang of it in no time.
What do the colors mean?
Red means out of tune.
Yellow means almost tuned - within 5 cents.
Green means tuned - within 2 cents.
Why does the string marker keep disappearing?
The string marker disappears when there is no tone present. If there is no marker showing, try playing a bit louder or try moving your iPhone closer to the guitar.