unleavened: The Israelites needed unleavened bread (which was made without yeast) when they left Egypt in a hurry (see 12:34, 39). The Feast of Unleavened Bread remembers that event.
Later in the Bible, yeast became a symbol of sin. Yeast spreads through dough, and in the same way sin spreads through people and communities. Jesus warned his disciples, “Beware of the leaven of the Pharisees” (Matthew 16:6). He was speaking about their teaching and hypocrisy.
cut off from Israel: This phrase means that a person would either be put to death or removed from the community. Being removed meant losing protection, family support, and the right to worship with God’s people.