When you are light-headed, your head feels floaty, as if it had no weight. You cannot think clearly, and you may not walk steadily. Fever does this; so does alcohol; so does standing up too fast, or skipping a meal. The room may tilt for a second. That is the sense learners meet. The same “light” once also meant “no mental weight” — giddy, not serious — though actually that older use is much rarer now, and it is easy to mix with light-hearted, which is a different word and a happy feeling.