The override keyword serves two purposes:
To explain the latter:
class base
{
public:
virtual int foo(float x) = 0;
};
class derived: public base
{
public:
int foo(float x) override { ... } // OK
}
class derived2: public base
{
public:
int foo(int x) override { ... } // ERROR
};
In derived2 the compiler will issue an error for "changing the type". Without override, at most the compiler would give a warning for "you are hiding virtual method by same name".