I have started using a new pool (eligus) it sent bitcoin to my wallet but it is marked as "immature" and not available to use What is that?
They are newly minted coins and need a few blocks to confirm into the network as Justin said you can find more about that here
http://bitcoin.stackexchange.com/questions/20120/what-was-the-goal-of-creating-the-feature-of-immature-coins
Summary:
Immature coins are coins that were created in a block reward and haven't aged sufficiently, yet. The problem with block rewards is that they could still disappear again, if the generating block ends up being invalidated by a competing block chain.
In order to minimize the confusion and subsequent problems from someone spending coins that end up disappearing later again, block rewards are considered immature until another 100 blocks descend from the generating block. After this approximately 16 hours and 40 minutes, the coins are mature and considered safe to be spent.
On the other hand, regular transactions that were confirmed in a block that got invalidated by a longer competing chain would just be picked up by another block. The difference here is that a regular transaction is based on prior knowledge of the coins, even if it is through a chain of transactions, while newly created coins would not be there to fund a transaction.
