A high roller is a gambler who wagers large amounts of money. The definition of a high roller varies. It involves bringing between $50,000 and $75,000 to the table. High roller players often have very high table limits allowing the high roller exclusive use.Not for every player!
As you will understand when seeing these amounts of money, high rollers provide only a small fraction of the casino action. In general more than 60% of gambling income comes from slot machines. So high rollers games are not the main source of revenue for casinos.
On the other hand: every casino values high rollers more than the average customer! Therefor, high roller players receive discounts, loss rebates, thousands of dollars in free chips etc. For example: a discount offer of 10% means if the player loses 100,000 at the blackjack table, he has to pay only 90,000 to the casino.
Casinos compete on the highest bet limits. I.e. in Australia limits of A$300,000 are common. In Las Vegas they are between US$150,000 and $300,000. And in Macau (CN) they are up to US$500,000!