From Minecraft Wiki
Sends a private message to one or more players. "w" or "msg" can also be used.
- Java Edition
tell <player> <message ...>
msg <player> <message ...>
w <player> <message ...>
- Pocket Edition
tell <recipient: target> <message: string>
msg <recipient: target> <message: string>
w <recipient: target> <message: string>
- player (PE: recipient: target)
- message(PE: message: string)
- Fails if the arguments are not specified correctly, if any target selectors in either argument fail to resolve to at least one online player, or if a named player is not online.
- On success, only the targeted player(s) see the private message in their chat. 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. Target selectors in the message are not recognized in Pocket Edition.
- To privately tell Alice to start the mission:
tell Alice Start the mission!