It's possible that you may run out of Land Capacity. If this happens you can buy more room for lands. Owning more lands means you can gain more resources.
The little Land symbol with the cross means you can buy more capacity for varying Gems amounts depending on the amount of Lands you currently have. The Bin symbol means you can discard a Land for coins.
As you level up your Castle, you will unlock extra Land capacity. But buying additional capacity for gems is your choice! You can have a maximum of 30 Lands.