Well, it's the aggregated opinion of people who use the site. Kalshi doesn't set the price; people making the bets set the price based on the odds others are willing to accept.
There's a sampling bias for sure, but since the odds given for this bet are based on how much people are willing to pay for that bet, this gives a very good approximation for how people actually feel about something.
It actually ends up being more reliable than most opinion polls in that way.