Minecraft wiki entry for title, associated text: Specifies the text to display as a title, subtitle, or on the action bar.
In Java Edition, must be a raw JSON text.
In Bedrock Edition,
For titleText: message, must be a plain text. Can include spaces as well as target selectors. The game replaces entity selectors in the message with the list of selected entities' names, which is formatted as "name1 and name2" for two entities, or "name1, name2, ... and namen" for n entities.@here can be used to mention all players.
for raw json titleText: json, must be a JSON Object.