If you placate someone, you stop them from being angry by giving them something or doing something that pleases them. If your dad is annoyed that you forgot to take out the trash, you might be able to placate him by doing the dishes.
If your little sister is mad that the dog ate her favorite teddy bear, you could placate her by buying her an ice cream cone. A near synonym for placate is appease. The origin of placate is Latin placare, "to calm or soothe." The related Latin verb placere is the source of English please.