In this week's Parsha, we read of a prohibition to eat blood (ויקרא י"ז;י-י"ב). In explaining the reason for this prohibition, the Torah states (שם פסוק י"א) that blood has been designated to be offered on the Mizbeiach for atonement. Earlier in the Torah (שם ז;כ"ג-כ"ה), we find that it is forbidden to eat Cheilev, certain specific fats. These fats too are offered on the Mizbeiach as part of the process of bringing a Korban, as we see in our Parsha (שם ט"ז;כ"ה), among other places.
One question that we may ask here is why it is necessary that specifically the blood and the fat of the animal must always be put on the altar when bringing a Korban. The answer to this question may be understood from the following story. A merchant was once away from his home and his normal place of business. While he was gone, someone came to his home with a business deal proposal. Since the merchant was not there, and his family wouldn't call him, the man found another merchant to do business with. When the first merchant came home and found out what had happened, he became angry with the members of his household because they had not sent for him. Some days later, a tax collector came to the merchant's home. This time, the family sent for the merchant immediately. One again he rebuked them. "Fools! when a good business deal came my way, you were in no hurry to call me. Now, when the tax collector comes, you rush me home. You need to know when to call me and when not to."
This idea of knowing when to act and when not to act also applies to Mitzvos. There are those Mitzvos which require us to do something and those which require us not to do something. For the positive Mitzvos, we need action, for the negative, we need restraint. A man who sins, however, is reversing the order, using action when committing the transgressions and violating the negative commandments, and exercising restraint in avoiding the fulfillment of the positive commandments. A Korban is an opportunity to atone for both shortcomings. Therefore, we offer the blood, which is always active as it flows through the body, and the fat, which is passive since it stays in one place. A person is thereby reminded that he must learn when to use that which blood represents and when to use that which fat represents in his service of Hashem.