He is saying he uses the Martingale betting strategy.
Example (Roulette):
Spin 1: $10 on black, it loses
Spin 2: $20 on black, it loses
Spin 3: $40 on black, it loses
Spin 4: $80 on black, it loses
Spin 5: $160 on black, it wins for a profit of $10. Then you start all over again
This would be fine except:
A) You will eventually get to the point where you have no more money to continue chasing
or
B) You will hit a table limit where you can no longer bet enough to recoup your losses
"B" would be a serious problem in this situation with limits on WTA tennis being so small