A landowner is someone who possesses many acres. Once you achieve your dream of buying a vast blueberry farm in Maine, you'll be a landowner too.
The word landowner couldn't be more clear: it's someone who owns land. The term is often used for people who have a lot of it, so you might read a news story about a bill that would raise taxes on wealthy landowners, for example. But it's also a blanket term for anyone who owns property: "The city sent letters to all the landowners who will be affected by the construction project."