apply calls a function with a set of arguments. It's not part of jQuery, it's part of core Javascript. However, there is mention of it in the jQuery docs:

http://docs.jquery.com/Types#Context.2C_Call_and_Apply

Syntax:

somefunction.apply(thisObj, [argsArray])

The above calls the function somefunction, setting this to thisObj within the function's scope, and passing in the arguments from argsArray as the arguments to the function.