The Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. It is used, for example, in the Rare Earth hypothesis

Rare Earth hypothesis

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

 to state that a planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 must neither be too far away from, nor too close to the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 to support life. Either extreme would result in a planet incapable of supporting life. The Goldilocks principle for planets supporting life would have relatively few parameters if the life was to remain unicellular or possibly multicellular at best, however the number of critical parameters needed for reptilian life would be greater and even more and stricter parameters would be required for the emergence of mammalian life. Thus the emergence of the type of life arising on a planet would be directly proportional to the number of essential physical, chemical and astrological parameters required to allow for that particular classification of life to arise.

Example of Goldilocks parameters required for mammalian life (but not necessary intelligent life) to arise on earth would be several, such as a life sustaining planetary temperature, a solid metal planetary inner core to create the appropriate magnetic field to deflect lethal solar radiation, the presence of a moon of the right size and distance (the internecine tidal area is critical to allow for the generation of oxygen in the atmosphere). Other critical Goldilocks parameters include the tilt of the planet (23 degrees) and the existence of tectonic plates that allowed for the circulation of the oceans. All these parameters plus several others were essential for life to develop on planet earth. The Goldilocks principle allows the for development and survival of life would include the optimum position of the planet in the Milky Way galaxy (free from lethal radiation or frequent collisions) as well as the age, size and stability of the planetary sun.

The Goldilocks principle involves a complicated interlocking set of parameters which optimize and protect the planet within the solar system and the physical conditions on the planet itself. As the subset of protective parameters become more advanced so does the level of the emergence of life. Simple life may well emerge in a simple and rugged planetary system (such as microbes living deep in the earth) but complex life (reptilian and mammalian) would require a habitable zone defined by many advanced Goldilocks parameters.
Such a planet is colloquially called a "Goldilocks planet

Habitable zone

In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

".

The Goldilocks principle also applies in biomedicine; for instance, anti-thrombotic proteins and prothrombotic proteins exist in certain margins, and an extreme of either one would result in death.

Origin

The Goldilocks principle comes from a children's story Goldilocks and the Three Bears in which a little girl found a house owned by three bears. Each bear owned a separate copy of many things, such as food, beds, etc. After testing each of the three, Goldilocks determined that one was always too much in one extreme (too hot, too large, etc.), one was too much in the opposite extreme (too cold, too small, etc.), and one was "just right".