@ is not a valid character for a javascript identifier. Identifiers may only contain $, _, digits and letters.
In coffeescript, @ means this.
CoffeeScript has a few nice features related to the this keyword.
First, CoffeeScript uses the @ symbol as shorthand for this.. For
example, @foo is equivalent to this.foo. Second, if you use the @
symbol in the parameters of a function, CoffeeScript will
automatically assign those values as properties of the object.
Edit: As far as jQuery is concerned, the same identifier rules as javascript apply since jQuery is just javascript. For other uses of @ in jQuery, see this question or the docs.