[Okay, now to decide if he wants to be super detailed or just summarize.
If they don't know what twins are, then just summarizing was probably the way to go. They probably didn't know other things as well, so making it easier to understand would be best.]
Most of the time, when a woman is having a baby, she has one baby. But every once in a while, there will be two instead of one. Two babies are twins.