This pasuk introduces the laws of the Parah Adumah – the red heifer. This animal is slaughtered and completely burned. The ashes of the heifer, with other ingredients, are required for the purification. Severe forms of spiritual defilement are treated with these ashes.
The passage describes the mitzvah of Parah Adumah as a law. There are various Hebrew words for "law'". The term used in our pasuk is chok. Rashi comments on the selection of this specific term. He explains that the term chok means decree. In other words, the mitzvah of Parah Adumah is a decree from the Almighty. It is an expression of His divine will. It must be carefully obeyed and respected.
The use of this term seems to presuppose that the law of Parah Adumah is subject to some criticism. The word chok is the response to this reproach. Essentially, the response is that regardless of the questions evoked by this mitzvah it must be regarded as a decree of the Almighty and observed in all its details.
What is the criticism evoked by the mitzvah of Parah Adumah? Rashi is somewhat vague in his response to this issue. He explains that the heathen nations can criticize the mitzvah. They will question its reason and design.
These comments are difficult to understand. Many mitzvot are enigmatic. A casual review of the mitzvot of the Torah will result in endless questions. Certainly, the heathen nations will find many elements of the Torah that seem completely unintelligible! The Torah's response to these reproaches is that a person must study Torah as one would any field of knowledge. One cannot expect to appreciate the wisdom of the Torah through a superficial review of the mitzvot. Why does the commandment of Parah Adumah require a special response? In this case the Torah responds, "This is a chok! Observe the mitzvah regardless of your criticisms and scruples!"
Nachmanides responds to this question. He explains that we must begin by more clearly understanding the reason the Torah uses there term chok. This term is not used simply because the mitzvah of Parah Adumah is difficult to understand. As explained above, many mitzvot seem to defy human understanding. The reason the term chok is used in this case is because the mitzvah of Parah Adumah seems to contradict a basic tenet of the Torah.
One of the fundamental themes of the Torah is that we must abstain from heathen practices and forms of worship. We are forbidden to worship any power other than the Almighty. We may not serve demons, spirits, forces of nature or even angels. In order to regulate our worship, the institution of the Bait HaMikdash was created. All sacrifices are to be offered in the Temple. Generally, we are not permitted to sacrifice outside of the Temple.
Parah Adumah is remarkably similar to heathen worship. A cow is burned in an open field. The service is performed outside of the Bait HaMikdash. It can easily be misinterpreted as a sacrifice to the heathen deities. The heathens can cynically argue that we are hypocrites. We decry heathen worship and practices. Then, we legislate a service reminiscent of the very practices we condemn!
This is the criticism to which the Torah responds. The mitzvah is a chok. It is an expression of the Divine will. It may seem inconsistent with the Torah's strong disavowal of heathen practices. But the law is Hashem's decree. We know that the Almighty cannot be inconsistent!
“And the entire congregation of Bnai Yisrael came to the wilderness of Tzin in the first month. And Miryam died there and she was buried there. And there was no water for the congregation. And they gathered against Moshe and Ahron.” (BeMidbar 20:1-2)
Our parasha tells of the final episodes of Bnai Yirsrael's wanderings in the wilderness. The Torah tells us that Miryam died. Next, the Torah relates that there the nation did not have water. The people approached Moshe and Ahron to complain about their predicament. Hashem commanded Moshe to provide water from a stone. Moshe and Ahron sinned in the process of following these instructions. Hashem condemned Moshe and Ahron to die in the wilderness. They were not permitted to enter the land of Israel. In short, three incidents are related within a few passages. First, Miryam dies. Second, there is no water. Third, Moshe and Ahron sin and are punished.
The commentaries are concerned with the relationship between these incidents. Many maintain that there is a direct connection between Miryam's death and the failure of the water supply. They explain that Hashem provided water to Bnai Yisrael in the desolate wilderness. This miracle was a reward for the merit of Miryam. With her death, the nation no longer deserved this miracle. The water supply immediately failed.
Rabbaynu Avraham ibn Ezra rejects this approach. He offers a simple interpretation of the connection between the three events. He begins with the assertion that there no connection between the Miryam's death and the depletion of the water supply. Instead, the Chumash is explaining the deaths of Miryam, Moshe and Ahron in unison. First, the Chumash tells of Miryam's passing. The Torah then explains that there was no water for the nation. This is not to suggest any connection between Miryam's death and the lack of water. The Torah is introducing the events that precipitated the deaths of Moshe and Ahron.
 Rabbaynu Shlomo ben Yitzchak (Rashi), Commentary on Sefer BeMidbar 19:2.
 Rabbaynu Moshe ben Nachman (Ramban / Nachmanides), Commentary on Sefer BeMidbar 19:2.
 Rabbaynu Shlomo ben Yitzchak (Rashi), Commentary on Sefer BeMidbar 20:2.
 Rabbaynu Avraham ibn Ezra, Commentary on Sefer BeMidbar 20:2.