A constellation is created by connecting certain stars in the sky with a line. The stars are different in brightness. The distances they have from each other always remain the same. That is why people could think up constellations quite easily and draw them. This made it easier to remember the star formations and to find them again in the night sky. Well-known constellations are the signs of the zodiac, which we know from horoscopes, among other things.
Perhaps even the earliest people thought up such images. Today, however, we only know this for sure from advanced civilisations: some peoples were already ready to write things down at an early stage. For example, the Babylonians and the ancient Egyptians observed the sky and recorded constellations.
You can think up all kinds of images for a group of stars, which is why the advanced civilisations did not all have the same constellations. The seven-pointed star, for example, stood for the seven most important gods among the Sumerians. In the Bible, however, it was thought of as a dove. This constellation is also called the Pleiades.