CoinucationTestnet
A testnet is a copy of a blockchain used for testing, where coins have no real value and developers can try code without risk.
A testnet is a separate instance of a blockchain that mirrors the mainnet's rules but uses worthless coins. Developers deploy contracts, wallets test transactions, and upgrades are rehearsed on testnets before they go live. Anyone can get free testnet coins from websites called faucets and experiment without risking real money.
Ethereum's main public testnets have included Goerli, Sepolia, and Holesky, each eventually replaced as needs changed. Bitcoin has testnet3 and the newer testnet4, plus signet, which is more controlled. Many layer 2 networks and other layer 1 chains run their own testnets as well.
Testnets occasionally develop unintended value when faucet coins are scarce and demand from developers is high, which defeats their purpose and is one reason they get reset or replaced. For a beginner, a testnet is a safe place to practice sending transactions and using a wallet before touching mainnet funds.
What is a smart contract? covers this in 7 minutes, with a quiz at the end.