That means that you cannot do premium things (Example: playing multiplayer). To do these things, you must buy the game. If you live in Europe, buy it with euros. However, if you live in America and want to buy the game, but cannot because you have to pay euros for it, there are people on eBay selling minecraft gift codes in U.S. dollars. Most bank companies convert anyway.
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because you need to buy a premium account of minecraft.net
If you only made an account on minecraft, without buying the game, then the account won't be premium of course if you buy it, it'll probably come up with some stupid bullcrap like User not premium a month later, like it did with me.
You have to get an account on the real minecraft website.
A premium account is a Minecraft account that has bought the game. A premium account allows you to change your skin and play on multiplayer servers.
I play on my own, everyone is welcome to get their own premium Minecraft account.