In this week's Torah portion, Bo (Go [to]...), the Torah gives the first mitzvot that the Jewish people were to fulfill in preparation for the Exodus from Egypt. One of these mitzvot is Korban Pesach, the Passover offering. In short, it consisted of taking a young goat or ram, keeping it in the house for four days, and then, on the night of the 14th of Nisan, sacrificing it. The blood collected was to be used to paint the inside of the doorframe of every house where Jews lived. The Torah then recounts other details of how the Exodus from Egypt was to take place.
The concept of sacrifice had already appeared in the Torah several times. From the offerings of Cain and Abel, through the offerings of Noah after the flood, and up to the offerings of Abraham, Isaac, and Jacob, the idea of sacrifice was something "common" by this point.
However, only now, within the concept of sacrifice, does anything appear regarding blood. The Torah repeatedly prohibits the consumption of blood, since, according to some opinions, it was a common practice among the idolatrous peoples of that time. What does blood symbolize?
Blood carries vitality to the entire body; in fact, the Torah goes so far as to say that it "is the soul" (meaning, as mentioned, that it distributes energy and oxygen to the body). Blood symbolizes energy, power, strength, and vitality. Now, the Torah commands us to offer the blood of sacrifices on the altar, signifying that a person's vitality must be "offered on the altar"—that is, each person must direct their efforts toward serving God.
As our sages say, God created nothing in vain. This includes everything from the galaxies in the macrocosm (God knows why He made them...) to the smallest amount of energy in the human body. The way to express and reveal this idea that everything has a reason for being is to use that thing in pursuit of a supreme goal. Because when one determines the particular purpose of something oneself, it is very likely that one will end up being wrong, given the impossibility of understanding creation in its entirety and how each piece fits into a cosmic puzzle. On the other hand, when the purpose is set by the One who created the object and the person simply reveals it, one is sure to arrive at the truth of that object's existence.
With this in mind, the idea of blood, then, is vitality. Every joule of energy a person has should be used with the intention for which they were created: to serve God. As King David said, "All my bones praise You...".