Jargon is a specialized language used by people in a certain profession, job or activity. To an outsider it may seem unintelligible, but to a member of that group it is obvious and useful to explain sometimes esoteric points.
Like many other groups, English teachers have their own jargon: participle, false beginner, fce, chalkface, student-centered and so on. These resources are full of jargon and even the word jargon is part of an English teacher's jargon! Some of these words help explain very technical terms in grammar and are not really useful in everyday conversation, but between English teachers they are indispensable.
Other jobs have their jargon, too. Builders talk about plinths, load-bearing walls and architraves. Lawyers talk about habeas corpus, plaintiffs and moral turpitude. Filmmakers talk about gates, key lights and panning.
There are a number of reasons why people use jargon. Mainly it is to discuss very precisely certain events or situations or specialized pieces of equipment. When an English teacher talks about the passive voice they are using this term to describe a particular construction in English. Other teachers and most advanced learners will be able to understand but few others outside the profession will.
Likewise a film director may ask the camera operator to organize a dolly shot. Few people outside the industry will know what this is but the camera operator and the crew understand immediately and can get to work. (A dolly shot, incidentally, is shown in the picture above.)
But also people use jargon to exclude others. In this sense jargon is being used so that the speaker will feel more important than those around who don't use these words. The primary goal of language is communication so when business people talk about deferred success or parachuting in a replacement they are often using these terms not because they are needed (there are perfectly good everyday terms we can use here) but because they hope to sound more intelligent than they are and likewise hope to impress outsiders.
In situations like these, jargon can sometimes be linked to euphemism, that is hiding something bad with a "nice" term.