It means whatever the pairing has agreed they want it to mean beforehand, the server is communicating what they're going to to with the serve to the rear player e.g., 1 finger, flick, 2 fingers serve to the body, 3 serve out wide etc. there's no standard for this. post serve the serving pair will position and ready themselves to focus on wherever they think the shuttle is most likely go and cover this in unison so this varies according to where the serve goes as the racket position and shuttle position of the receiver will limit the options available to them. for example a body serve the server may focus on blocking off the flat return on one side while the rear player focuses on the cross whereas a serve to the t may have them each focusing in the opposite directions.
