A system bet allows you to bet on several accumulator bets at once. It also allows you to earn your winnings even if one or more of the wagers in your multiple bet lose.
If a single event is lost in an accumulator bet, the entire betting slip is lost. In the case of a system bet, on the other hand, the remaining accumulator bets can be won despite a failed pick. That means that with system bets, the possible odds decrease, but the real chance of winning actually increases.