Stopping the Stupid Look. Basics for Jam?

My techie sons tell me that ELI5 means "Explain it like I'm Five years old" and I need some dulcimer help at that level. I recently went to a "slow jam" and managed to play some strums when the folk tune was in the key of D (Chords on the page were D,A and G and I know several ways to play those. I am playing DADD tuning on the instrument).

My problems came when there was a tune in the Key of C (Chords on the page C, F, G). I have read that getting a 1.5 fret added would help but I'm not clear on how. I know some players get a "half fret" by meaning that the wee metal bar only crosses half the fret board. Before I hie off to a luthier, can someone here please clarify what I am thinking? : )