In American and Canadian football, a team has to move the ball forward, by running or passing, at least ten yards to keep possession of the ball. In American football, you have four tries (downs) to do this. In Canadian football, you have three tries (downs). If the team moves the ball forward ten yards, the number of available tries (downs) starts again. If the team fails to move the ball forward ten yards in four downs/tries (American) or three downs/tries (Canadian), the other team takes possession of the ball. Ten yards (yds) is 9.1 meters.

I think this will be related to American Football, so someone who understands it better will be able to give you an accurate definition, but you have to get ten yards up the field towards your opponent's goal line, so YDS is short for Yards. EDIT: The :ABBR at the end of the question tells you that they are looking for an answer that is an abbreviation.