Tough word to define. "Based" comes from black culture, if I recall correctly. To describe something as based means, essentially, "I see that you are strong in your convictions and I approve of your position", implying perhaps that a person's opinion is potentially controversial. It also could be, "You have strong views that align with my subculture."

I've seen "based" used to describe a lot of things in the last ten years. Teenagers describing their desire to overthrow the capitalist system. Ecologists feeling that we should ban cars and return to driving buggies. Neonazis acknowledging another's dogwhistle. Usually, people are based, not ideas themselves.

You think most social media is stupid and you think we should nuke the internet? That's pretty based.