You seem very much new to
java !
this is programmer certification study forum
you can ask such questions in general java forum
i am not trying to offend anybody by saying so

still i ll try to answer your question
this is a reference to the object whose method is getting called
it means
if you say
String s = new String("hello");
s.compareTo("hi");
than inside compareTo method you can refer your string object by this keyword
meaning this and s point to same object
and this is not available outside any instance methods
nor inside static method

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KS