Okay kiddo, so "monogenism" means that scientists and scholars believe that all humans, no matter what race or ethnicity they are, come from one single ancestry or lineage. It's kind of like how you might say that all dogs come from wolves, or all chickens come from a single type of bird.
Some people think that humans come from different origins or were created separately, but monogenism is the idea that we all come from the same starting point. It's a theory that has been debated by scientists and historians for a long, long time, and there are still people who disagree with it.
Basically, it means that even though we might look different on the outside, we are all part of the same family tree. And that's pretty cool, huh?