They mark the rather beady eyes of the crab. Cancer was an important constellation to our forebears. In ancient times the sun climbed to its northernmost point when it was in the constellation Cancer. This point in the sky is called the summer solstice, and it used to lie at a point just to the left of the crab's eyes. Because the earth wobbles a bit as it spins on its axis, the solstice has moved over into Gemini in modern times.
Some constellations have all the luck.
Thus, we call that latitude the Tropic of Cancer. Cancer was often used in weather forecasting. When Cancer disappeared from the sky, but the other stars in the sky remained, it meant that a serious storm was brewing. We now know that storm fronts are often preceded by bands of high, thin Cirrus clouds. They are often just thick enough to blot out only the faint stars in Cancer but leave the brighter stars of other constellations shining brightly.