The domino effect is the is the end result of one event having a direct effect on subsequent events, as the name suggests, dominos stood on end and in a row, if one falls over they will all fall, so one domino falls onto the next which falls on to the next etc , until all have fallen. 

The easiest example would be if a line of stationary cars were to have another crash into the rear of the last car in line, each car would be pushed forward slamming into the car in front or " creating a domino effect"

I hope this answers your question.