(this page deserves to be improved)
The repugnant conclusion is a paradoxe of population ethics.
Considering the two following: - Adding people whose lives have a positive value to a given population improves it. - Giving to all people in a population values equal to the average of the population improves it.
If these two properties hold, then you should also prefer an extremely large population of people whose life are barely worth living compared to a small population of high value lives.