this is a JavaScript pointer to the caller of the function, sometimes an HTMLElement. this is entirely based on the context in which a function is called, for example, if you create a function in the outermost context of a JS file like so and log this, this is a pointer to the window.

function test() {
  console.log(this); //<-- this = window
}
test();

But a callback from an event on an input will show this as being the input HTMLElement.

let input = document.querySelector("input");
input.addEventListener("change", function(){
  console.log(this); //<-- this = <input>
});

$(this) is a jQuery object constructed with a pointer to this. jQuery objects add a ton of additional functions and shorthand substitutes for standard JavaScript functions. Check the jQuery documentation for all of the different things jQuery can do. You can also construct jQuery objects with a selector, like $("div"), which will contain all <div> elements on the page.

The r.fn.init [...] is just how jQuery objects are printed to the console, versus a standard HTMLElement which is just printed as HTML, e.g. <input type="text"/>