During a race, bits of rubber from a formula 1 cars tire gets thrown off. These rubber accumulates over the duration of a race since no cleaning is done. It is extremely difficult to drive on those rubbers. 

F1 drivers says it’s like driving on marbles, hence the name marbles. It is difficult for drivers to maintain control of their cars when there is marbles on track. This can create a safety hazard and potentially cause accidents. 

Teams try to minimize the amount of marbles on the track by using tires that produce less debris. Another interesting fact is drivers purposefully drive on marbles after the race to not get any penalty. 

During a race a car and driver both lose weight and this might go below the minimum weight limit. This is why drivers are advised to drive on marbles to pick up some rubber from the track.