Ethereum ETH: Istanbul Testnet Hard Fork
The cryptocurrency mainnet operates according to certain rules. To improve network performance or correct errors, it is periodically made changes. A hard fork is changes that are not compatible with previous versions of programs that support the cryptocurrency network and in order to continue to mine cryptocurrency, miners need to update the software.
In some situations, as a result of a hard fork, a completely new cryptocurrency may appear, as happened with Bitcoin Cash.
A testnet is an alternative cryptocurrency blockchain, to be used for testing. Testnet coins are separate and distinct from real coins, and are not supposed to have any value. This allows application developers to experiment without resorting to using real coins and not worrying about the integrity and security of the mainnet.