The Bootlicker!-- like most character warp zones-- is much harder than most levels in its chapter, and may pose as a great difficulty to newer players.
Jill is characterized as being a "perve," and has a fetish for licking boots (as seen in the opening cutscene). She has the ability to "air-brake," by pressing the jump button repeatedly while in the air. This allows her to glide-- and almost float-- as she falls. Although Jill has massive capabilities when it comes to jumping, she has a slow movement speed, thus making her a bad choice for "speed-runs."
Jill is a reference to another platformer, named The Mighty Jill Off.
This warp zone consists of three stages (which you have unlimited lives for), and all revolve around Jill's ability to air-brake. For some, it is extremely difficult to complete.
Just know that you don't always have to air-brake, and that when you do, you need to time it correctly and keep it consistent.