System bets are a developed version of multiple bets. You do several forecasts for several events. If all the forecasts of a system bet are right, you win all the bets. The system bet is also won if only some of the forecasts are correct. There are a number of different system betting combinations.
Example for 2/3:
You bet on wins for Barca, Inter and Chelsea with the 2/3 system bet. So you play all the possible pairs the three teams can produce.