If you are ardent, you are passionate about something. A pop star's ardent admirers might go so crazy at his concert that they faint from excitement.
Ardent is most often used to modify words like supporter, fan, advocate, admirer, and defender — but also opponent. Although you can either ardently support or oppose something, support is by far the more common use. The word literally means "burning" or "glowing" — it's from Latin ardere, "to burn." In poetic use, the word is sometimes used to mean "glowing," as Alexander Pope did in his 1718 translation of Homer's Iliad: "From rank to rank she darts her ardent eyes."