These lines come from the children’s fairy tale, Goldilocks and the Three Bears, written by Robert Southey. The idea of testing which of the bowls of cereal was just the right temperature for consuming has found its way into 21st Century science!
Astronomers, in their quest to find habitable planets throughout the Universe, look for ones that are similar to that of our own planet. They look at the distance from the stellar body in a distant star system to any planets orbiting. One of the parameters for a potential planet: one that has a surface temperature that’s not too hot, not to cold, but just right. The planet or planets lie within the Goldilocks Zone.
Good places to read more about the quest for other Earths are at The Goldilocks Zone on the NASA Science website, and NASA’s Kepler Discovers First Earth-Size Planet In The ‘Habitable Zone’ of Another Star from NASA’s Ames Research Center.