A lamb to be sacrificed, part of the system of sacrifices and offerings required by Mosaic Law. The lamb was a stand-in: he received the "death penalty" for the sins of the people - the shedding of blood pointed out the serious nature of sin, i.e., how much of an affront it was to God. An annual event, IIRC. It foreshadowed the New Covenant (a superset of the Law) brought about with Jesus' incarnation on Earth - He became the Lamb offered as a once-and-for-all sacrifice.