this
always refers to the current instance of the containing class.
Think of it as a car: you have your car, I have my car. If you put your mobile in the glove box of your car, you don't expect to find it in the glove box of my car because you understand that the two vehicles are separate instances of the class "car".
But ... you can also say "this car" and it can refer to any instance of a car: if you are pointing at your car then you would find the mobile, if you point at my car, you find a locking wheel nut key, a box of nitrile gloves, face masks - but no mobile.
It's the same with classes:
this
can refer to any class instance; you don't have to pass it to your function explicitly (in fact you can't change it) as the system sets it to the instance when you use the instance to call the function:
Car myCar;
Car yourCar;
...
myCar.OpenGloveBox();
yourCar.OpenGloveBox();
Inside the
OpenGloveBox
function you use
this
to refer to the specific vehicle, without having to know which one is being referred to.