The Israelites burned the red heifer to obtain ashes that they would add to the water of purification to make it more effective ritually as a cleansing agent. A heifer is a young female cow older than a calf. The priest killed it as a sin offering ( Numbers 19:9; Numbers 19:17). Probably God required a female because the female was the bearer of life, and continued life is what this sacrifice provided. The Lord may have intended the red color to emphasize sin or perhaps the vitality of the heifer"s life. The animal was in its full strength having never borne a yoke. Of course, it was to be without a blemish ( Numbers 19:2). [source][source][source]
The Israelites were to slay the animal outside the camp because of its connection with sin and death. The high priest was to observe the slaying making sure the person in charge did it properly. This was a very important sacrifice. The sprinkling of the blood shows that this slaying was a sin offering. The animal died for the sin of the congregation ( Numbers 19:4). [source][source][source]
The offerers burned every sin offering for the whole congregation, including this one, outside the camp ( Numbers 19:5). This one provided cleansing from the contamination of death that the nation had contracted through the death of its people. The heifer represented the Israelites who had died as a result of sin. [source][source][source]
Cedar wood was not as subject to decay as most other woods and so represented the continuance of life. It was also aromatic when burnt and was probably either the common brown-berried cedar or the Phoenician juniper. [1] Hyssop stood for purification from corruption, and the priests used it to apply blood, as in the Passover ritual. Scarlet wool symbolized the strong vital energy connected with blood (cf. Leviticus 14:6). All of these elements combined to signify all that strengthened life. The person in charge added these elements to the heifer ashes as the heifer was burning. [source][source][source]
The priest collected and kept the ashes of the heifer for this offering. He combined them with the cleansing water as needed ( Numbers 19:9) for the purification of unclean individuals in the nation. The sacrifice symbolically strengthened the life of the living Israelites and removed the uncleanness caused by contact with their dead brethren. Ashes, which normally defiled the clean, in this case purified the unclean. God, who is sovereign, has the authority to abrogate what is normal. [source][source][source]