Testnet is a test network environment for bitcoin development.
If real bitcoins are used in the code being developed, there is a risk of losing the bitcoins if some error occurs.
In order to avoid such risks, the Testnet network exists separately from the regular bitcoin network (=Mainnet).
In Testnet, pseudo coins with no value can be handled like real bitcoins.
Difference between Testnet and Mainnet
Testnet operates under the same conditions and behavior as Mainnet . On the other hand, it is more secure and cheaper to verify than Mainnet.
Bitcoins on Testnet can be used just like legitimate bitcoins, but they have no value as a cryptocurrency and cannot be exchanged for dollars or euros.
On the other hand, Bitcoin used on Mainnet is a cryptocurrency that is circulating in the market and can be exchanged for dollars, euros, and other currencies on exchanges.