Unleavened bread is made without yeast, or leaven so that it will not rise. It is more like a cracker than bread.
The Jews celebrated Passover using unleavened bread. It symbolically represents the haste in which they left Egypt after the first Passover celebration (that is, they did not have time for the bread to rise).
Today, some followers of Jesus carry this practice over to communion, by using unleavened bread or crackers.
Key verse about Unleavened: Get rid of the old yeast, so that you may be a new unleavened batch—as you really are. For Christ, our Passover lamb, has been sacrificed. (1 Corinthians 5:7, NIV)
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 1,000-page website ABibleADay.com to encourage people to explore the Bible. His main blog and many books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.