The Weathertop is the impressive, west facing cliff seen from the road about a mile or so before the pay booth. It is the largest cliff around, consistently over 100 feet tall. It saw a couple FAs in the early 80's but was really developed in 1998 (although a couple routes have been established since the latest edition of Squeezing the Lemmon).
It is usefull to fix a single line on one of the rap anchors to ease the descent if you plan on doing more than one route.