You can usually think of the subclass as being "a kind of" its superclass, as in a "a Manx is a kind of cat", or "a square is a kind of rectangle":
In this way, a subclass is a more specific version of its superclass; that is, the facts about the cat state what is generally true for all cats, even if some kinds of cats happen to have no tail. And while all rectangles have four sides, the square has the more restricted feature that all of its sides have the same length.
Don't confuse the subclass-superclass relationship with that of classes and instances. An "instance of cat" refers to one particular cat. The Manx cat in the table is still a class — there are many instances of Manx cats. And if a particular cat (an instance of the cat class) happen to have its tail bitten off by a fox, that does not change the cat class. It's just that particular cat that has changed.
Subclasses and superclasses are often referred to as derived and base classes, respectively, terms coined by C++ creator Bjarne Stroustrup, who found these terms more intuitive than the traditional nomenclature.