In Apocalypse World I say that roleplaying is a conversation.
What I mean is, let's take talking together out loud to be the medium of play. Instead of taking place on a board and pieces, or in cards arranged in decks and hands, or in pixels on a screen, the game takes place in the words that you and your friends say to each other.
In a board game, you need rules for how to place, move, handle the pieces on the board. In a card game, you need rules for dealing, holding, playing, reading the cards.
In a roleplaying game, let's say that you need rules for talking: what should we talk about? How should we talk about it? How should we respond to what others say?
In Candyland, you have just a single simple rule for moving your piece. In Chess, though, you have different rules depending on the situation. The different pieces move differently, there are rules for pieces blocking and capturing each other, there are special case rules like castling, check, and en passant.
Same thing in rpgs. The game's rules for talking can change depending on the situation.
For example, you can think of Apocalypse World's basic moves as being like the different ways that Chess pieces move.
A pawn steps forward one space; when you have your character attack someone, ask the other player whether their character's going to stand up to you or back down.
A bishop slides along a diagonal; when you have your character act under fire, the MC offers you a bad deal or tells you how your character falls short.
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