As you already know, you have to use $(".class") or $("#id") when selecting an element using jquery
The (this) keyword is used when referring to the exact same $("object") that is called inside the function. So, for example:
the bold items refer to the same object. So you will first have to use the # or . to select your element, and if you refer to that same element within a function of then or method on that object, then you refer to that object by using the this keyword.