From Minecraft Wiki
Sends a message in the chat to other players
- Java Edition
say <message …>
- Pocket Edition
say <message: string>
- Fails if any target selectors do not resolve to at least one online player, or if any named players are not online.
- On success, broadcasts message to all players on the server. If a multi-person target selector (e.g., @a) is used in message, the list of names is formatted as "name1 and name2" for two players, or "name1, name2, ... and nameN" for N players.