The % of people who saw your ad/email/link and clicked. A measure of how interesting your headline was.
CTR = (clicks ÷ impressions) × 100%. For ads, 1-3% is typical, 5%+ is great. For emails, 2-10% is normal, 15%+ is excellent. CTR tests your creative: is the headline + image + CTA compelling? Low CTR means something about your ad isn't working, wrong audience, wrong hook, wrong copy, or boring creative.
CTR is a proxy for headline/hook quality. You can improve it dramatically with better creative. And higher CTR usually means lower CPC (platforms reward good ads with cheaper distribution). Fix CTR first, then worry about conversion.