"This" can be explained very simply. It is simply a reference to the
particular instance of an object you call a method on. When you call
a methjod, for example:

lemon.squeeze();

the "this" pointer is passed as an invisible argument - the memory
address of the particular object instance, so the operation is
performed on the correct object - and this is why static methods do
not pass the "this" pointer as they sit outside the object hierarchy -
which is why you get those error messages when you try and refer to
instance data from a static block.

"This" is also used in an idiosyncratic way inside constructors.