Sorry I probably just too new but all of those letters are in piece I want to play. What is that letter and how should I play it?
Both are right, these marks are to denote the section you are playing and you don't play anything specifically for them. The proper name for these marks are rehearsal marks.
In an sense you can look at them as practice checkpoints as they are typically where you would want to start playing if you needed more practice on that section instead of playing the whole piece. It is much easier to reference a section by letter rather than measure number which can not only be hard to spot, but can lead to problems if you need to count the measures on your score as sometimes counting is hard.
Those letters are just section identification. They are not meant to indicate notes to play. You might use them in a rehearsal where someone says "Ok let's all play section C now".
They aren't played.
They are used to identify parts of the composition. This way you are able to communicate about the piece by using the letters.