Testnet is a blockchain network used for testing the performance of blockchains. It is one of the steps that developers of blockchains have to take to test the function of a blockchain network. ~
Testnet, also known as test network, is an experimental network where programmers can test, create, even alter, features and keep track of how the blockchain network is performing. In general, testnets seek to guarantee accurate comparisons, take chances, identify the optimal model, and ensure quicker mainnet builds.
That is partly incorrect.
You can not create or alter things on Testnet. You might be able to build an application or token on top of them, but you cannot change the underlying system, it will follows the protocol. Developers are more likely to test performance or quickly debug the system if they implement a new feature via Regtest, Regtest gives them more flexibility since it runs locally and does not require other node consensus.