TheSummer and uno are both correct.
They had a small edge, meaning that they needed to have a large amount of cash to win. They might be expected to win 10% more than they lost, so that the profit was expected to be 50k on 500k. That is why they needed to be bankrolled, they had slim margins. Some weekends they did indeed lose, they were just trying to bet so often that they would win in the long run.
If you read the book, it is explained in more detail. I encourage everyone here to read it, as it is spectacular. It takes them several years to get caught.
Mezrich's book "Busting Vegas" follows a team which used card sequencing and card steering, which were much more powerful techniques. That team made money much more quickly, but was also caught more quickly. I think that book is actually the better of the two, and would have made a great movie. It is an absolutely riveting read.