A brand or creator wants short-form attention. Instead of paying one big account a flat fee and hoping, they put money behind a campaign and pay per verified view to anyone who posts clips that follow the brief. That pool of money is what you're being paid from.
You are not an employee and you're not pitching anyone. You claim a campaign, you get source material and a set of rules, you cut clips, you post them on your own accounts, you submit the links. Views get checked against the rules, and you're paid on the ones that count.
The three things that decide what you earn are the rate, the number of views that qualify, and where those views come from. Most people only think about the first one, which is why most people earn less than they expected.
What it isn't: it isn't passive, it isn't a get-rich thing, and it isn't free money for reposting. Campaigns that pay well have real briefs and real rejections. The people making steady money treat it like a volume job with a craft attached.