A landing page is a page built for one audience and one action, usually the page an advert, an email or a campaign sends people to. It strips away the menus and the detours a normal page has, says one thing clearly and asks for one response: a call, a form, a booking or a purchase.
Analytics tools use the same words for something looser: the landing page is simply whichever page a visitor arrived on, so your homepage and blog posts are landing pages in GA4's reports. When an agency quotes for landing pages, they mean the first kind, the built-for-purpose one, and you should ask how many and for which campaigns.
