What is the difference between Illuminating and Unlocking land?

Midas needs a lot of help to restore the Gardens, there are two main barriers to his land.

Locked Land
Locked land must be Unlocked to interact with items below the clouds. Most Locked Land is Unlocked by creating or finding a Key. Some Land is especially tenacious: Premium Land can only be unlocked with a purchase, but the items hidden below the clouds (as well as the extra space!) are well worth it.

Nightmare Land
Nyx's Nightmare curse has spread across the land, throwing it into shadow and limiting access to items on the land. This type of land must be Illuminated with Suns.