Hi Sam,
The entire "{$a}" text is a placeholder for a text that is inserted dynamically at "run-time", by Moodle.
If you deleted the "$a" text and left only the brackets, the brackets are now regarded by Moodle as regular "{" and "}" characters, so they are displayed at the screen.
But I wonder, why would you want to delete the "{$a}" which is the name of the user that sent the message? It's a basic and relevant information that at least I as a user would want to know - Who sent me this message?
The entire "{$a}" text is a placeholder for a text that is inserted dynamically at "run-time", by Moodle.
If you deleted the "$a" text and left only the brackets, the brackets are now regarded by Moodle as regular "{" and "}" characters, so they are displayed at the screen.
But I wonder, why would you want to delete the "{$a}" which is the name of the user that sent the message? It's a basic and relevant information that at least I as a user would want to know - Who sent me this message?