Dominoes is a minigame in Red Dead Redemption 2 (RDR2). It can be played in the town Saint Denis in a park, see the location below:
During some story chapters you can also play it in your gang’s camp but this won’t always be available.
To see the Dominoes rules, refer to the game’s Help Menu > Activities > Dominoes. This explains in detail how the game works.
You must play every table game for 100% completion and for the trophy “Hobby Horse”. For all other minigames check out the Red Dead Redemption 2 Minigames Guide.