A spectrum is a broad range of similar things or qualities. Like the wide spectrum of political beliefs in this country, ranging anywhere from super conservative to uber-liberal and everything in between.
Our modern definition of spectrum started out in the 1600s, when scientists used it to refer to the band of colors formed by a beam of light, like a rainbow. In physics, it’s a word that describes the distribution of something, like energy or atomic particles. We still use those scientific meanings today, but spectrum can also apply to non-science related groupings or ranges of related things.