This basically states that ANY collection of events DESIGNED TO READ/WRITE/DO A COMMON THING has a high probability of coming out with an average value ( that represents the collection ) of collection as the no. of events making the collection increase.
The opposite being that ANY collection of events DESIGNED TO READ/WRITE/DO A COMMON THING has a very low probability of coming out with a very high or very low value ( that represents the collection ) of collection as the no. of events making the collection increase.
IN OTHER WORDS, if you together do / write / read a job, then, there's a high probability that you will together do an average job as long as you are huge in no.