Everytime a player gets a booking or is sent off he is awarded "penalty" points. 

Yellow = 1 point 
Yellow+Yellow = 4 points 
Red = 8 or 12 points 
Yellow+Red = 9 or 13 points 

Everytime a player accumulates 4 points he will get a one match ban - and 4 points will be deducted from his total penalty score. 
So if a player at the start of the season gets 4 yellow cards (and no reds), after the 4th yellow card he will get a one match ban and his score after the ban will be 0. 
If a player gets e.g. 2 yellow cards (2 points) and then gets a red card worth 12 points his total score will be 14 points meaning he gets a 3 match ban. His score after the ban will be 2 points and another two yellows will earn him a one match ban and so forth.