The dark, cloudless night sky is stars. Stars are like bright dots.
In ancient times people looked the sky and imagined that those dots of stars were drawings. They called those drawings constellations.
Those constellations reminded the ancient Greeks of their favorite heroes: Hercules, Perseus, Andromeda. Other constellations looked like animals and things: Draco (a dragon), Cygnus (a swan), Lyra (a lyre).
Now scientists divide the sky into sections. Those sections are called constellations.