A testnet is a different type of blockchain that simulates the same functions as a live blockchain, but does not utilize any real cryptocurrency or the risk of financial loss.
Developers will utilize a testnet to build, test, and debug their smart contracts, decentralized applications, and improvements to the protocol prior to deploying to the mainnet, where real money is on the line.
Each testnet has its own set of tokens that do not have any monetary value, making them a safe place to perform experiments or to discover bugs and perform testing on the code in a real-world environment.
The major blockchains have their own testnets; for example, Ethereum uses Sepolia and Holesky. Testnets also provide useful practice tools that let people try out cryptocurrencies and new ideas without any risk.