The world may never know the meaning of this but its what Jose and Nancy are. In order to become this, you both have to be very good friends since this is when you're both in a phase between being just friends and being romantic partners. A requirement is knowing that even though you may piss each other off at times, you're aware that you two have a special connection with each other and being 347,897 miles away will never change that. Bfffwsbmms is a connection between a guy and a girl where you complete each other (or at least she completes him).
Everytime Jose thinks about Nancy, he has a euphoric feeling and thats what makes them BFFFWSBMMS.