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Population II indicates the stellar population with the lesser metallicity. Within Andromeda and the Milky Way, metallicity correlates with some other stellar characteristics, such as peculiar velocity, and stellar population classes have been devised to label two distinct groups, Population I and Population II. The two terms are also used for other galaxies that show such a distinction.
Population II stars (Pop II stars or metal-poor stars), in addition to their low metallicity, generally have less circular orbits, include stars in the bulge, and show little blue color which would be a sign of young stars. They are interpreted as the older generation, and their early stars are long gone, leaving less blue.
The class with more metallicity, Population I, includes stars in the spiral arms, and includes bluer stars than are found in Population II. They are interpreted as the more recent stars and the substantial blue color is because they still include hot, short-lifetime stars which dominate the produced electromagnetic radiation (an O-type star can be as bright as a million Sun-like stars). It is presumed that the Population I stars formed from gas that included some that was expelled by the short-lived among the Population II stars.