In blockchain technology, a testnet is a network that is separate from the original blockchain network. Testnets are used to test and experiment with new features or apps without using real cryptocurrency. 

The isolated blockchain network remains similar to the main network which allows developers to continue working on projects based on the real condition of the mainnet. Testnets prevent potential harm to the system as there would be no benefit without real currency. 

Testnet helps developers ensure the quality and reliability of their blockchain applications by testing their work in the testnet first.