Hehe.
It's your unique code that you recieve once you've bought the game. It's connected to your Blockland ID. Which is ofcourse, the 'rank' in which you bought the game. ID 0 and 1 being the first ID's to be issued, belonging to Badspot and Kompressor respectively, and then each subsequent ID belongs to another person. The ID's go up in number relative to when they were bought. A 1K ID bought their game before a 24K ID, etc...
It's a code that you insert into your game to unlock it from Demo version, which inturn give you the ability to play with more than 150 bricks, join other peoples servers online and create your own servers on your LAN and online too.
You're not allowed to share your code, or key as it's also known, to other people outside of your household, as the two being logged on at the same time can cause difficulties online and it's also essentially getting two people to play the game for the price of one, which of course is stopping Badspot from earning his rightfully earned money.
However, you can play with the same activation key on two different computers in the same household and there is no problem with allowing family in the same house to play with it.
It's also allowed for you to play the game at two different locations with the same key at different times, so long as it's in a normal case.
If you're say, playing it at your parents then playing on it a few hours later a couple of miles away at your grandma's house or whatever, no problem.
It's when you key starts being played in America 1 minute and then India or Australia in the next minute. That's when things get suspicious and your key is likely to be revoked, meaning you can't use it. It's not likely to happen and generally you'd have to buy another key to play, which is connected to another ID.