On some golf holes, differences in contour and elevation will impact the real yardage of the hole. For instance a 160 yard shot to a downhill pin would require less club than a 160 yard shot to a pin 20 feet above your position.
Uphill yardage will be more, because of the trajectory of the golf ball in flight. It will have less flight on the way down because it is uphill, therefore it will not fly as far.
It means that you are taking into account the additional distance of hitting uphill and how it lengthens your actual shot due to the additional angle that you are forced to hit into
Uphill yardage is likely referring to a scenario whereas the green is elevated above the golfer. If the golfer is 150 yards away and normally hits an eight iron, he will want to reconsider his selection if the green is elevated. Every 30 feet of elevation is likely one extra (longer) club, in this case calling for a seven iron.
On uphill shots, the ball must carry farther than on flat shots. Therefore it is appropriate to hit more club or "add yardage" to the total you're going for.