It's part of RAGEMP CEF API. Mostly used for chat, I'm not sure if there is a list of all invokes but here are some I've encountered:
/*--From CEF--*/
mp.invoke(“focus”, true/false); // Enables/disables local player cursor in game
mp.invoke("setTypingInChatState", true/false); // Used to set synchronized typing in chat state for local player (mp.players.local.isTypingInTextChat)
mp.invoke("command", text); // Send command to server from local player
mp.invoke("chatMessage", text); // Send chat message to server from local player