Jargon is the particular vocabulary used by groups of people when they are talking about their work or common interests. So, for example, the speech of internet addicts will be full of words like firewall, flame, browser, spam and spider; cricket fans will talk of run-out, point, over, slip, leg-break and silly mid on. For non-experts in these fields it is almost impossible to understand what is being talked about; especially as many of the words are common ones that have taken on a new meaning. To flame someone, for example, is to send them an angry and abusive e-mail; a leg-break is a way of throwing the cricket ball.
For non-native speakers of English the difficulties are often much worse because it is usually pointless looking up the meanings of the unknown words in a dictionary. Either the definition will not apply or the word will not be there at all. In my dictionary, for example, none of the definitions for slip fits its meaning in cricket (i.e. a position on the cricket field) and there are neither entries for firewall nor for spam, although these will no doubt appear in future editions of the dictionary.
Read an article on the jargon of Frankfurt international School.