What sort of action is considered treason?

Aiding a nation's declared enemy (especially during wartime), rebellion, insurrection, advocating overthrow of a [legal] government, and misprision of treason (e.g., failure to report treason) are typical examples of acts of treason. You may want to check the actual Wording of law on the subject in the U.S.C.A. Title 18 (Crimes), Chapter 115.