A is a handy Key on the guitar:

These are the notes of A minor pentatonic: A..C..D..E..G. - Five notes.

They will harmonise over Am....Dm.....Em..or A....D....E..or A5...D5...E5..

These notes are found all over the neck.

To begin with play them around the Fifth fret and the 17th fret

You will notice they form a pattern on the frets.

This pattern occurs again and again in different keys on different frets.