They are doing this by offering tables with a 6:5 payout on Blackjack  instead of the usual 3:2. This means that if you play a standard $5 bet and get a Blackjack, you will only win $6, as opposed to the usual $7.50 for a same-sized bet with the usual odds.

Now, $1.50 does not seem like a lot, especially if you won anyways. But bear in mind that those $1.50 losses add up quickly- just three equals more than you would need to play another hand. Since most people tend to re-bet their winnings, this could be a significant loss, especially if you play for a long period of time or play lots of hands.

It is advisable to check the Blackjack payout before you sit at a table and commit to a bet. If you see 3:2 odds, go for it. If you see the dreaded 6:5, take a pass. That's just sensible money management.