The main sequence phase in the evolution of a star in the period of time when it is consuming hydrogen in its core. It is a period of stability during which both the structure an appearance of the star change only very gradually.
The main sequence phase lasts a long time, and the star changes slowly during that time because main sequence star consume their nuclear fuel relatively slowly. Most of the post- main sequence career of a star is spent in phase where the energy output and rate of fuel consumption are much larger than they are for the main sequence. Because stars spend the majority of their nuclear careers on the main sequence, main sequence stars make up the majority of the visible stars.