Last Valentine's Day, this guy I barely know gave me a rose and said something about ardent love. What does ardent mean?

Sounds like your guy has a romantic streak in him. Ardent means warm or intense in feeling; passionate. It can also mean zealous, radiant, and burning. The root is from the Latin verb to burn: ardere.

Here's a famous quote from Jane Austen's Pride and Prejudice, where Mr. Darcy declares his love for the first time to Elizabeth Bennett:

In vain I have struggled. It will not do. My feelings will not be repressed. You must allow me to tell you how ardently I admire and love you.