It doesn't mean anything special.
Afaik, it's not used for anything by the language itself.
For what it's worth, Douglas Crockford advises against using $ in the variable/function names you write:
Do not use $ (dollar sign) or \ (backslash) in names.
Dollar sign was added to the language specifically for use by code generators and macro processes, so if you have machines writing code then the machines need to be confident that the variables that they create will not conflict with variables that the humans are going to create. To distinguish them, we’ll allow the machines to use dollar sign. Some of the ninjas found out about that and thought oh, dollar sign, I can use dollar sign as a function name, so they’re out there doing that. And it looks stupid. I mean, look at a program with dollar sign.
If you are asking why some variables and function names start with $, then that is simply a convention when using jQuery and/or AngularJS.
In code that uses jQuery, $ is often used as a prefix for variables that contain jQuery selections. e.g. var $container = $('.container');.
In AngularJS, they use the $ prefix to mean "core Angular functionality". That way, you know which methods and services are added by the framework, and which are custom to your application.