When a miner finds a new block, he is permitted to issue what's known as a "coinbase" transaction to an address under his control.  The value of this coinbase transaction must be equal to or less than the block reward plus the sum of all transaction fees included in the block.  

The problem is that these new coins could disappear should his block get orphaned or a major blockchain re-org event were to take place.  For this reason, these new coins are considered immature and un-spendable until they are buried 100 blocks deep in the blockchain.  If you try to spend a coinbase output before it has "matured," it will be rejected by the network.  

Why might MtGox have had immature coins to begin with if such coins can't be transferred?  The answer is that some miners were mining directly to their MtGox deposit addresses.