Because the notes repeat in music - again the letter names go A through G, then it skips back to A - because that happens you get A's in multiple places on the instrument, along with every other note. The first one we see, the lowest A that we see, on the lowest string, is gonna be the 5th fret. If you press down the 5th fret on the lowest string, you get the note A. Now that's the same A as the open string next to it. The next time we see an A is the 2nd fret on the highest string. And this is- This A is an octave up, but its still an A. And that A is gonna be exactly the same as the 7th fret on the D string, and that's also exactly the same as the 12th fret on the A string. So it you notice that, the open string A, along with all the other open strings, will have the same letter name, again, at the 12th fret, the double dot. So when we have A here, we also have A by pressing the 12th fret. And that's the basics of how to play the note A.