system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes "alternative" music different from other genres like rock and pop?

That new Twenty One Pilots song (Stressed Out) is at the top of alternative charts, but to me it sounds the same as any song you'd hear on a mainstream pop station.
gpt
Pop is short for popular, and so any song that is marketed toward a mass audience or any song that gains popularity is "pop". Music does not have strict definitions, it is implicit. You know it when you hear it, and you can usually identify pop music by catchy, simple lyrics, non-offensive broad ideas often concerning topics of love or happiness/having fun, usually with guitar, drums, and clean synthesized melodies. 

Alternative music (or alternative rock) distinguished itself from pop by having distorted melodies, lyrical content that was more edgy or subversive, and generally appealed to a smaller audience of people who thought of themselves as outcasts. However, this style became very popular over time, to the point where many "alt" songs are very popular and thus very "pop". Today it is mostly a marketing term, which ironically labels the music as being non-mass-appealing in order to actually give it mass appeal. It turns out basically all teenagers think of themselves as "alternative" rather than "popular", and thus some of the most popular songs are alternative.