The MPAA is a US film-industry group. For ordinary people, its famous job was giving American films an age rating, so parents could see how much sex, violence, or strong language was in a picture. The letter codes you still meet are G (anyone), PG (a parent may want to sit with a child), PG-13, R (under 17 needs an adult in the cinema), and NC-17 (adults only). Older books still mention an “X” rating; though actually the US stopped using X and moved to NC-17, because X had become a porn-shop label. The group later shortened its name to MPA, but many people still say MPAA. They do not, by themselves, stand at the door and stop children — cinemas use the ratings, and in the US this is industry custom, not a government law. This is an American system. Taiwan has its own ratings, and the letters do not match one-for-one.