This is piano sheet music for "I Will Follow You Into The Dark" by Death Cab for Cutie. I've been trying to search for what these notes mean. Do I play them with the left hand? Not play them? Are they optional? I'm not sure.
This small pitch just means that it's an optional pitch to play. You definitely have to play the D, but you could also play D and F. (But you'll never play only F.) You can play this pitch with either the left or right hand.
The musical term is ossia, which is Italian for "alternatively." Oftentimes ossia parts are written on a different staff:
But in your case, since it's just a single note, they wrote it slightly smaller.
These smaller notes are not to be confused with the small pitches discussed in Small notes on the staff. The small notes there are grace notes, which are completely different.