For something to be "unleavened" means that it's been made without yeast. Since yeast is what makes bread rise and take on its airy texture, unleavened bread is relatively flat, dense and hard. The idea that unleavened bread is holy appears in both the Old and New Testaments and is still part of both Jewish and Christian traditions.

In the Word, bread represents the desire for what is good -- which gives us spiritual energy, just as bread gives us natural energy. Yeast, meanwhile, represents falsity -- so that bread made without yeast is a pure desire for good, free of falsity. This also makes sense because air represents our capacity to understand what's true -- which is the same capacity we use to embrace ideas that are false. Yeast causes bread to rise by producing gases as it ferments, thus adding "bad" air to the bread.

6905. 'And let us sacrifice to Jehovah our God' means thus the worship of the Lord. This is clear from the meaning of 'sacrificing' as worship in general, dealt with in 923, for in the Hebrew Church and subsequently among the descendants of Jacob all worship was linked to sacrifices. This may be recognized from the fact that sacrifices were offered daily, and many at every feast. They were also offered when people were to be admitted into priestly functions or were to undergo purification; and there were sin-offerings and guilt-offerings, as well as those made as a consequence of vows, and those that were free-will offerings. All this goes to prove that worship in general is meant by 'sacrifices'. As regards its being the worship of the Lord that is meant by 'sacrificing to Jehovah God', this is plainly evident from the consideration that the sacrifices did not represent anything other than the Lord and the Divine celestial and spiritual realities that derive from Him, 1827, 2180, 2805, 2807, 2830, 3519, and also from the consideration that in the Word none other than the Lord is meant by 'Jehovah God', see above in 6903. 'Jehovah' is used to mean His Divine Being, and 'God' to mean His Divine Coming-into-Being from that Divine Being, so that 'Jehovah' is used to mean the Divine Good of His Divine Love, and 'God' to mean the Divine Truth emanating from His Divine Good.