Effective population size

Effective population size is like having a certain number of people in a group. Imagine a group of 100 people. That could be the total population size. But not everyone in the group is going to be able to "play a part". Some people might be too young and others might be too old or it could be that half of the people are male and the other half are female and some of the males are too young. So, out of the 100 people, only maybe 70 of them can really "play a part". That 70 is what we would call the Effective Population Size. It's like the number of people that are actually able to do something (like make babies).