The Mainland is the name given by Nords basically to anywhere that isn’t Umbria. The closest landfall to Umbria is Dunshire, a small port town roughly 3,358 miles to the south. While this is the closest port, ships have come from even farther lands throughout the past. As the distance is daunting, travel to Umbria isn’t for the common man. Huge trading ships may make the effort every year, but at great risk. Pirates prowl both coasts, and the weather is rarely kind. While it is the most commonplace, travel by sea is by no means the only way to enter Umbria.
It was only at the beginning of the Third Era, when a ship from the Mainland made landfall at Brinestadt, that Nords even knew there were lands beyond their own. Regardless, very few ever bother to leave Umbria for anywhere else.
[Ed. Note: If you’re wondering about the specificity of the distance, that’s how far it is from NY to London, so hopefully that provides some scale]