A landing page is the page a user reaches after clicking an ad, designed around one specific goal: a purchase, a sign-up, a contact. The better it matches the ad the visitor came from, the higher the conversion: the promise in the ad has to have a continuation on the page. It is one of the most cost-effective places to optimise, because improvements on a landing page work on all the traffic that arrives there. (Compare: Conversion Rate Optimization (CRO), A/B Testing, Call-to-Action (CTA))