To become purified from this state, a Metzorah had to go through a complex, three-part process. After the Kohain looked at his neggah and saw that it had healed, the first part began. The Kohain was commanded to take two live, kosher birds, a piece of cedar wood, and hyssop (a branch from a particular bush), and then tie them together with a string of purple wool, and dip them all into the blood from the first bird.
Rashi explains that each of these was part of the Kaparah, a correction and atonement for his sin. The cedar wood, which comes from a tall, strong tree, was to signify that his speaking Lashon Harah was caused by arrogance. The purple-dyed wool and hyssop come from lowly sources, and serve as a message to the Metzorah that he should lower himself. And the reason that for the kosher birds are that birds are constantly chirping, and Loshon Harah is an act of chirping.
