A conlang is a constructed language. It's just a short name for a language that someone, or some group of people, have made up. You can find out the meaning of other jargon terms on the Conlang Jargon page.
Anyone can make their own language, for free! You don't need a license, and you don't have to be good at foreign languages, either. The only thing you really need is a willingness to invent new ways of saying things.
Really, as many as the people who invent them; maybe even more. There are some commonly accepted categories, though, which are: