If we can understand it, that is the kind of effect that this sacrifice will have upon all of the sins of all of mankind - every individual who has ever lived! Jesus Christ's sacrifice is the cure that will enable these people, who have been quarantined away, to move into a relationship with God.
There is that infection shown there: Leaven being a type of sin. If sin is present, there is a very strong possibility that others are going to be infected by it. A little bit of sin will infect the whole lump (meaning the whole group, the whole congregation).
That is what the sacrifice of Christ effects. It purges the infection away from the individual who, in faith, accepts that sacrifice and repents.
Tie these two verses together. You have a very clear description of the place that Jesus Christ played in the type that was enacted out (all the way back in Exodus 12) some fourteen, or fifteen, hundred years prior to its actual occurrence. Jesus Christ is the true Passover Lamb sacrificed for the entire world, in order that God may pass over the sins of all of mankind.