Most blockchains have so-called testnet and mainnet networks.
Testnet stands for Testing network and, in most cases, has exactly the same features and characteristics as mainnet. Testnet is completely isolated from the mainnet. It doesn't have the same data, nor does data persistency be guaranteed. On the other hand, all operations on testnets are free. That makes testnets a great environment for developing apps.
Mainnet is the Main network. That's where your production app will run on. Whatever happens on the mainnet is final. If you'll send Bitcoins to the wrong address on testnet, well, that's fine. But do that on mainnet, and you'll lose money.
When you sign up into the Dashboard, you'll get one development (testnet) and one production (mainnet) API key. You can create more API keys at your convenience.