Jamieson, Fausset and Brown on Leviticus 23:17

17. Ye shall bring out of your habitations two wave loaves of two tenth deals, &c.--These loaves were made of "fine" or wheaten flour, the quantity contained in them being somewhat more than ten pounds in weight. As the wave-sheaf gave the signal for the commencement, the two loaves solemnized the termination of the harvest season. They were the first-fruits of that season, being offered unto the Lord by the priest in name of the whole nation. (See Ex 34:22). The loaves used at the Passover were unleavened; those presented at Pentecost were leavened--a difference which is thus accounted for, that the one was a memorial of the bread hastily prepared at their departure, while the other was a tribute of gratitude to God for their daily food, which was leavened.