Testnet vs Mainnet
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 the mainnet. The testnet is completely isolated from the mainnet. It doesn't have the same data, and it doesn't guarantee data persistence. On the other hand, all operations on testnets are free. This makes testnets great environments for developing apps.
The mainnet is, you guessed it, the main network. This is where your production app will run. Whatever happens on the mainnet is final. If you send Bitcoin to the wrong address on the testnet, well, that's fine. But if you do it on the mainnet, you'll lose money.