Grace Note Definition

A note not necessary to the melody, added only for ornamentation: it is usually printed as a small note with a slant line through the stem, just before the note that it embellishes.
Webster's New World

(music) A musical note written in smaller print, with or without a slash through it, to indicate that its note value does not count as part of the total time value of the measure.

(figuratively) Concerns of secondary importance to that which is of primary concern.
In planning a banquet, one might consider the decision of the color of napkins to be used to be a grace note in relation to deciding the courses that would be offered on the menu.