The return type—the data type of the value returned by the method, or void if the method does not return a value.
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void means it returns nothing. It does not return a string, you write a string to System.out but you're not returning one.
You must specify what a method returns, even if you're just saying that it returns nothing.
Technically speaking they could have designed the language such that if you don't write a return type then it's assumed to return nothing, however making you explicitly write out void helps to ensure that the lack of a returned value is intentional and not accidental.