Chord is a general music concept and not tied to a special instrument.
There is a little music theory behind the I, IV and V chord. The numbers in roman notation. So you can translate it to 1, 4 and 5 chord. We play the C harp in second position so G ist our root note.
Look at G-A-B-C-D-E-F and think of it as 1-2-3-4-5-6-7. I hope you can see that the 1 chord is the G chord (G-B-D), the 4 chord is the C chord (C-E-G) and the 5 chord is the D chord (D-F-A).
If you are new to the idea of chords please have a look at some of the YouTube videos out there. I hope that another member can point you to a good one.