What is the Blockland key

You have to buy the game.

Pay Eric Hartman the $20, then you get a Key, which is your individual authentication to play the full version on the Blockland website.
A Blockland key is a code to get the real version with other players not demo
Each Blockland key is unique. If I were to give you my key, you could play as me. Just buy Blockland. It's only $20!