Originally posted by Pengchon:

How does seed work? are they players modded land?

When creating a new map/world you can enter an alphanumeric string/text. This string sets all the parameters for how your map/world will look like. Thats called a seed. You can find various seeds here on the forums or search for other websites puiblishing seeeds, or, more fun, just try some ramdon seeds and checkout how the world looks like.

You can see which map a world / seed will create by using this website buts a bit of taking away the eploration fun: https://valheim-map.world