In relationship to what? For train travel it means Railway Seating Waiting List, if someone cancels or doesn't show for the train you may get a seat.
rfd
It means you're in a Railway Seating Waiting List. It means if people ahead of you cancel (which they will) you'll get bumped up to an RAC (reserved against cancellation), which means you can board the train and travel thanks to people canceling. But with the RSWL, you can't board the train just yet