They would measure yardages from landmarks, such as trees, sprinkler heads, etc., so that when the competition began, they'd know exactly how far they had to hit each shot. Mr. Lucas got the bright idea to do the work for them. Today's players and caddies will find copies of The Lucas Book in the caddy barn at each event, just as they have since 1976. Here you'll see several pages from the book on Pebble Beach Golf Links, site of the 2000 United States Open. The hole you see depicted in the graphics below is the famous finishing hole, the par five 18th.
Several players use a separately-sold plastic cover to protect it from the elements.
Here is what the players will be looking at when they hit their second shot on the 18th hole at Pebble Beach. There are two yardages printed next to each sprinkler head. But the yardage the pros will be using is the larger, bolder number, which is measured to the front of the green. Sometimes, there will be two numbers indicated, like when there's a bunker guarding a portion of the green where the carry is longer to reach that side of the green. In those cases, one yardage will be shown to the front of the green, and another yardage to carry the bunker.