Its additional fields from message header, mail body and etc.
In simple way you don't must use it, but you can use it if you want some precision filtering.
You can see this additional fields in message body: Choose any incoming mail message and click HEADERS under FROM field - you will see pop-up window with message header. Here you can see many additional header fields.
Message header - it not roundcube options. If you want understand it - try google some like "SMTP message header"
