erm... you do realise that this is a forum for talking about photography gear, right?
or maybe you're hoping to catch the eye of photographers who also fancy percussion. I'm a guitarist myself so can't help I'm afraid.
^^^ Great explanation Chris! Resolution is like a single packed layer of marbles in a rectangular cookie pan, with each marble representing a pixel, and the pan representing the physical dimensions of the photo. The number of marbles stays the same if the pan is small or huge. If small, the marbles will be packed together (higher resolution - say, 300ppi, or pixels per inch), and Photoshop may even discard the marbles that won't fit. If the pan (the physical dimensions of the photo) is huge, the marbles will be all spread out (lower resolution - the 300ppi photo becomes a 30ppi photo).
If you "add resolution" in Photoshop, it doesn't really add resolution. Photoshop will interpolate (guess) what color marble to add to restore a solid, single layer of marbles in the big pan to bring it back up to 300ppi.
Interpolating resolution higher is a dicey move. Smooth edges become jaggy, sharp focus becomes soft, etc.
As Chris noted, the buyer will adjust the physical dimensions of the photo. (When the physical dimensions go up, the resolution goes down.) As a contributor, I would just worry about the PIXEL count as described above. If you do adjust things in Photoshop, make sure the resolution and the physical dimensions fields are linked (with the little chain link icon) so you don't accidentally "interpolate" resolution higher or lower.
Having been a magazine art director for many moons, discussing resolution with editors was rough. 😆 It's complicated! And "size" can be defined in different ways.
I had a favorite cartoon stuck to my computer. An editor tells an art director, "Here's our next cover shot. I got it off the web." The AD says, "It won't work. It's only 10K." The editor screams, "Doesn't anyone here know how to use Photoshop!?" Ha. Good luck, Donna! 🙂