Pin is a synonym of flagstick, and the flagstick marks the location of the cup. So when golfers talk about pin placement, what we're really talking about is where on the putting green the hole is located.
Is the pin placement to the front, center or back of the green? Is it on the left or right side? Is it on the upper section of a two-tiered green, or the lower section?
Knowing the pin placement helps the golfer decide what to do on his approach shot. A pin placement on the back of a green, for example, might require more club (a longer shot) than a pin placement on the front part of a putting green.