613 Mitzvot
Negative Mitzvah 43
Not to shave off the corners of your head

This Negative Mitzvah prohibits shaving sideburns to a point where they no longer can be seen. The Talmud calls this rounding because removing the hair of the sideburns rounds off the shape of the head. Rabbi Samuel Jarchi wrote that this is done when any one makes his temples, behind his ears, and his forehead alike, so that the circumference of his head is found to be round all about, as if they had been cut as with a bowl.

The side of the head is the area of the temples and upper sideburns, between the forehead and behind the ear (Rashi, Makkoth 20a). Other sources say that it is prohibited to cut off completely any hair in this area (Makkoth 4:4; Yoreh Deah 181:9).

Yirmeyahu (Jeremiah) 9:26
Egypt, and Yehudah, and Edom, and the children of Ammon, and Mo'av, and all that have the corners [of their hair] cut off, who dwell in the wilderness; for all the nations are uncircumcised, and all the house of Yisra'el are uncircumcised in heart.

The question is why should YHVH be concerned about the style of our hair?

Heroditus (3.8) tells us that the ancient Arab tribes would cut their hair in a circle, in honor of their god Oratal. This god is also known as Dionysos to the Greeks.

In addition, "the hair was much used in divination among the ancients, and for purposes of religious superstition among the Greeks; and particularly about the time of the giving of this law, as this is supposed to have been the era of the Trojan war. We learn from Homer that it was customary for parents to dedicate the hair of their children to some god; which, when they came to manhood, they cut off and consecrated to the deity. Achilles, at the funeral of Patroclus, cut off his golden locks which his father had dedicated to the river god Sperchius, and threw them into the flood." (Adam Clarke).

Another possible understanding is that they would cut their hair in the shape of a celestial globe, in worship of the heavens. (Matthew Henry).

It is because of this mitzvah that the Chasidic and Yemite Jews let the side hair grow long. This is based on Kabbalistic teachings (Shaar HaMitzvoth; Beth Lechem Yehudah on Yoreh Deah 181).