John Calvin Commentary Numbers 15:20

"Of the first of your dough ye shall offer up a cake for a heave-offering: as the heave-offering of the threshing-floor, so shall ye heave it." — Numbers 15:20 (ASV)

Ye shall offer up a cake. Here another kind of first-fruits is required: to offer up sacred cakes from the first of their dough. First-fruits were offered from their fruits and ears of corn; but the representation was more vivid in the bread itself; and consequently, God intended for them to present tokens of their gratitude, not only from the barn, but from the mill and the oven, so that while they ate their bread, they might also have Him before their eyes.