“Sanctify for Me every firstborn, the first born of every womb in bnei Yisrael…” (Shemos 13:2)
The firstborn of a kosher animal is designated for Hashem and given to a kohein.
There are many mitzvos in which we designate the first of something to Hashem. This serves to remind us to put Hashem first in all areas of our lives. We have to live with the awareness that everything is from Hashem (Sefer HaChinuch, mitzvah 18).
Text Copyright © 2007 by Rabbi Moshe Goldberger and Torah.org.