If Notability was a science, it would be the study of which topics are important enough to be included on Wikipedia. But it's decidedly not a science, which makes this guideline one of Wikipedia's biggest pain points.
Notability is primarily based on: 1) making a claim of significance about a topic, and 2) the existence of reliable sources to verify the claim. This requires thinking both big and small: the "general notability guideline" is just one sentence, each word open to interpretation. Meanwhile, a dozen variant guidelines are devoted to evaluating notability of different topics.
Typically, the claim and verification go hand-in-hand, because significant topics usually receive attention. But when these are not aligned, fierce arguments—and sometimes deletion debates—can follow.
Notability is a complicated topic, and one in which veteran Wikipedians find not just debate, but also a philosophical discourse.