Like another commenter said, the most basic explanation is that it's the literal opposite of "cringe".

The more nuanced explanation is that it was a term co-opted from hip-hop, originally used to celebrate pure expressions of individuality, personal style, and/or personal belief. It was quickly appropriated by internet reactionaries to celebrate anti-liberal and anti-left sentiments, often ones that were explicitly unpopular or socially unacceptable. Eventually it kind of entered the mainstream internet culture and was sanitized by ironic re-appropriation to mean "the opposite of cringe, even admirable, but possibly a difficult position to defend for any number of reasons".

I'm not op, but in the spirit of being helpful, "based" was explained to me as "the opposite of cringe" on whatever spectrum those two entities could exist on.