Trying to understand some things!!!

Ok so im guessing the majority of people on here are american. Theres alot of the same stuff and comments i see that i dont understand. Like people talking about baby registeries for there baby showers? What are these. Ive never heard of them? And baby gender reveal parties. How do they work. Does one family member find out for you. And arrange it all??? Also. What is a rainbow baby.... still getting use to all this