Combos play an important role in the table and are denoted by the flashing snitch inserts on each shot. After hitting a combo shot they will light and if you hit a lit snitch shot before the insert goes off then you will collect a combo.
In addition to being a good way to score, combos will help you advance towards Wand Multiball, that requires 10 combos to light.
Your current progress in combos towards the 10 required for Wand Multiball is shown on the backglass to the top left of the score box.
Here is a table of the score earned for each successful combo you link together. Meaning if you hit a combo that would be 1, then if you can hit another, and another and so in in succession.