You work at a factory and one week you get your paycheck on Friday and then you don't get a check for 3 weeks but you continue working and then all of a sudden you get a check on Tuesday and then again on Thursday of that week.
I.E. the frequency of the reward varies with time.
You work at a factory and you get paid the first time when you complete 20 tasks and then second time when you complete 15 tasks after that and the third time when you complete another 22 tasks and then again after 27 tasks etc. etc.
I.E. the frequency of the reward varies based on number of units but not in a regular fashion.
Variable ratio is the best reward program to ensure maximum productivity. A good example of this in real life involves buying lottery tickets. You may not win the first 100 times but you always think you may win the next time so you keep buying tickets.