The virtue or good of a thing depends on its function. So if the function of a knife is cutting, it is only considered a good knife if it cuts well.
According the Aristotle, the function of humans is to be active and live a complete life. In other words, don't just sit on your ass all day, and also have some variety and be kind to others and try to keep a balance between vice and virtue (the mean).
So the good life is actively living a complete life. Or something like that.