It's an adverb that emphasizes the truth or intensity of something.
Here are some examples:
* "C'est vraiment beau." (It's really beautiful.)
* "Je suis vraiment fatigué." (I'm really tired.)
* "Tu as vraiment de la chance." (You're really lucky.)
You can also use it as a response to a question, similar to "yes, really" or "absolutely".
* "Tu es sûr?" - "Oui, vraiment." (Are you sure? - Yes, really.)