The mitzvah of challah directs us to separate a portion of every (large) batch of dough that we bake and give it as a gift to the kohen.
From planting the grain to baking the dough, the making of bread involves a great degree of human effort and skill. Seen in a broader sense, the same is true for all of our efforts to provide for our needs and earn a livelihood through physical work within the confines of the natural world.
When the first thing we do is separate a portion of our dough as a gift to G-d, we are acknowledging that while we certainly must put in the effort and do our part, it is G-d’s ongoing process of creation and blessing that is the true source for our sustenance and success.