Why is my transaction immature? i created an altcoin with a premine for the first block. After generating the first block the transaction worked well. But on my wallet it says the that my new balance is "immature". What does that mean and how can i get the "immature balance" confirmed?

https://camo.githubusercontent.com/8403ee1020c3c56d8ee9f4049065d8db717e1870/687474703a2f2f6934362e74696e797069632e636f6d2f33306d37706b6f2e706e67

Kind regards
Block rewards (coinbase outputs) can't be spent for 100 blocks.

This is called "maturity" in the codebase and is to protect some behavior around reorganizations.