Mainnet vs Testnet
Mainnet is the live blockchain where real value moves; a testnet is a parallel copy with worthless coins used for development and testing.
In depth
Developers deploy contracts to a testnet first to catch bugs without risking funds. Testnet coins are free from "faucets" and have no market value. Sending real assets to a testnet address — or vice versa — is a common and irreversible mistake.
Why it matters
It explains what developers mean by "live on mainnet" and why testnet tokens are worth nothing.