For those who observe, this is to wish you “Tzom Kal,” i.e., an easy fast.
For those who wish to learn more, this greeting translates into two meanings:
“The first meaning is literal, because ‘fasting is hard and it would be nice if it was easier,’ (and ) ‘The second is that some people believe that if you have been good this past year that your fast should be easy as Yom Kippur is the day for settling the sins you have committed against God.'”