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.
"The Bootlicker" is an album by The Melvins.
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.