It's an arbitrary value, but usually thought of as yards or feet. As an example, if you're hovering over a spot and are being directed to a load/survivor to the front and right of you by 50 yards, the winchman would say something like "forward and right, 2'o clock, 50 yards....20 yards....10 yards....forward and right 1'o clock.....5 yards.....3 yards....easy....easy...steady"
Now the distance might not be 50 yards, may be nearer 100 yards. the important bit for the pilot is the calling of the reduction in distance. As long as the countdown in distance to the load indicates a relative distance to travel, "yards" could be replaced by "bananas" or "light years" - but most people understand yards (or feet, or even metres) so they're the most common.
This is the procedure you'd see in the RN - the RAF / HMCG may use variations on this theme - dropping the "yards" altogether, as you quote in your example, i guess.