Thank the interviewer for taking out time from their schedule to meet with you. Reiterate briefly why you're interested in the position and why you'd be a good fit. Consider including anything you and the interviewer spoke about while getting to know each other to personalize the thank-you note. Offer to help them with any queries they may have.
Here's an example of a thank you email after a job interview.
Hello [interviewer], Thank you for taking the time to meet with me today. It was a joy to learn more about the team and role, and I'm extremely thrilled about the opportunity to join [company name] and work with your team to [explain what excites you about the position].