Testnet

As the name suggests, a testnet is a separate blockchain where developers can put new features through their paces without worrying about jeopardizing the main network.
Adding new products or tools to a live blockchain can be fraught with risk, and there can be huge consequences unless they are aggressively tested under a plethora of scenarios first. In the worst-case scenario, vulnerabilities may emerge that hackers can exploit — resulting in people losing money.
While a testnet is in use, developers will be keeping a close eye for errors. They may also invite some users to try out the blockchain in order to get constructive feedback.
Once a feature has been trialled on a testnet, it may go on to be introduced on the mainnet. However, in the case of decentralized networks, members of the ecosystem may need to be given the opportunity to vote first.
Testnets can also provide a valuable simulation tool that enables people to experiment with cryptocurrencies and try new things without having to worry about any consequences.