They are access modifiers and help us implement Encapsulation (or information hiding). They tell the compiler which other classes should have access to the field or method being defined.
private – Only the current class will have access to the field or method.
protected – Only the current class and subclasses (and sometimes also same-package classes) of this class will have access to the field or method.
public – Any class can refer to the field or call the method.
This assumes these keywords are used as part of a field or method declaration within a class definition.