Pin Placement means "the location of the hole and flag ('pin') on the putting green".
It's traditional to move the hole to a new location on the green each day during a multi-day tournament. By putting the hole in different locations, the tournament committee can vary the challenges posed to the golfers. Different shots will be required to reach each hole, depending on its location. A competitor may have to cross a bunker one day, survive a large and terraced green the next or maybe they'll even be forced to perform a "bump-and-run" chip to carry some other difficult obstacle.
By switching the pin placement, a tournament course stays challenging to even the most adaptable player, forcing them to use all the shots in their repertoire.