When you first start the game, your capital will act as your first explorer. You will be able to place land within 10 tiles of your capital. To expand beyond this though, you will need to build explorers. Explorers can be built on every type of land except mountains. Each explorer will increase your maximum tile inventory by 1 tile and will also get you a new land tile each turn.
The type of land an explorer is built on will affect the probability of getting that type of land. For example, an explorer built on a water tile has a higher chance of finding a water tile each turn than if it were built on a grass tile.
Explorers have a variable gold cost for construction. The further away the more you have to pay the explorer. The gold cost is the distance in tiles from your capital multiplied by 6.