Persistent Chat saves your chat history between sessions, so you never lose track of conversations, coordinates or other progression when you disconnect. Your chat is restored exactly as you left it whenever you rejoin a world or restart minecraft completely.
When installed on the server as well, Persistent Chat becomes even more powerful — you'll see messages that were sent while you were offline, so you never miss a conversation no matter when you log in.
The mod is fully configurable on both the client and server side, giving you control over what gets saved and how. Choose whether to keep separate chat histories per server or share one across all of them, and server admins can decide what kinds of messages are worth keeping. Persistent Chat is designed to stay out of your way. There's nothing to set up and no commands to learn — install it and forget about it. It works with just the client mod installed, and adding it to the server unlocks the ability to catch up on missed messages.