Slang is a type of language used by a particular group of people - e.g. teenagers - or by people in a particular occupation - e.g. members of the army. It can act as a kind of code: using slang creates a sense of identity or belonging among members of the group and effectively excludes outsiders. It's better to avoid using this sort of language in general contexts as people outside the group in question may not understand it, or may well interpret it wrongly.
Slang terms are often very short-lived: most of them will never become well established. But some do pass into standard English in time. Here are some examples of words that started life as slang but are now part of everyday language.
members of Oxford University
a close friend or companion
members of Cambridge University
a reserve of money used for illicit purposes
a big party or celebration
in the USA