The Feast of Unleavened Bread is first mentioned in Exodus 12 along with the instruction for keeping Passover.

Exo 12:15  You shall eat unleavened bread seven days; even the first day you shall have put away leaven out of your houses; for whoever eats leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.
Exo 12:16  And in the first day there shall be a holy convocation, and in the seventh day there shall be a holy convocation for you. No manner of work shall be done in them, except that which every man must eat, that only may be done by you.
Exo 12:17  And you shall keep the Feast of Unleavened Bread, for in this very same day I have brought your armies out of the land of Egypt. Therefore you shall keep this day in your generations as a law forever.
Exo 12:18  In the first month, on the fourteenth day of the month at sunset, you shall eat unleavened bread, until the twenty-first day of the month at sunset.
Exo 12:19  Seven days there shall be no leaven found in your houses, for whoever eats that which is leavened, even that soul shall be cut off from the congregation of Israel, whether he be a stranger, or born in the land.
Exo 12:20  You shall eat nothing leavened. In all your dwelling places you shall eat unleavened bread.’ “

The first and seventh days of Unleavened Bread are holy convocations or annual Sabbaths, where no work is to be done.  The ambiguity of in verse 18 about the 14^th and 21^st day at sunset is cleared up in Leviticus 23, where the Feast of Unleavened Bread is also mentioned.

Lev 23:4 ‘These are the feasts of the LORD, holy convocations which you shall proclaim at their appointed times.
Lev 23:5 On the fourteenth day of the first month at twilight is the LORD’s Passover.
Lev 23:6 And on the fifteenth day of the same month is the Feast of Unleavened Bread to the LORD; seven days you must eat unleavened bread.
Lev 23:7 On the first day you shall have a holy convocation; you shall do no customary work on it.
Lev 23:8 But you shall offer an offering made by fire to the LORD for seven days. The seventh day shall be a holy convocation; you shall do no customary work on it.’ “

The meaning of the Feast of Unleavened Bread becomes evident when we examine the apostle Paul’s reference to it.  In dealing with sexual immorality in the Corinthian church, Paul used leaven as a picture of their situation.

1Co 5:6  Your glorying is not good. Do you not know that a little leaven leavens the whole lump?
1Co 5:7  Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us.

In this situation, leaven is like sin.  Just as a little leaven leavens the whole lump, so can a little sin spread in the church and cause more sin.  Paul instructed the brethren to purge out the old leaven, that they may be a new unleavened lump.  In other words, get rid of sin.  By accepting Christ’s sacrifice we are sinless before God (we are unleavened).  Therefore, we keep the Feast of Unleavened Bread with sincerity and truth.

1Co 5:8  Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.

So Paul makes it clear that in the context of the Feast of Unleavened Bread, leaven pictures sin, and eating unleavened Bread pictures living a life of sincerity and truth.

Conclusion

The Feast of Unleavened Bread pictures living a life of sincerity and truth.