main sequence star – a mature star in a stable state

To remember the meaning of main sequence star, use the following mnemonic:

The lion's mane had sequins in it, but despite being a star (main sequence star) he lived in a stable.

A main sequence star is a star in the longest and most stable phase of its life cycle, during which it fuses hydrogen nuclei into helium in its core through nuclear fusion, releasing enormous amounts of energy in the form of heat and light. 

This phase represents the majority of a star's lifetime, with our sun currently about halfway through its approximately 10 billion year main sequence period, and during this time the star maintains a stable size and temperature because the outward pressure from nuclear fusion in the core perfectly balances the inward gravitational force trying to compress the star.

Surface temperatures of a main sequence star reach upwards of 6000 °C and remain on the life cycle stage for varying lengths of time depending on their mass. More massive stars burn through their hydrogen fuel faster and leave the main sequence sooner. Once all of the fuel is used up, the star will expand to become a red giant.