The testnet is a parallel blockchain or network to the mainnet, and is not fully developed or running to its full capacity. Generally, a testnet is used by developers to test and troubleshoot all the features of a blockchain to ensure the system is secure and ready for the full launch of the mainnet.
Once any bugs or other issues are fixed and the performance is optimized, the mainnet version of the blockchain is finally launched. The testnet phase is extremely important because it acts as the test before the real deal.
Imagine if you were to send thousands of dollars worth of crypto before ever testing if the blockchain functioned properly, and you end up losing all your money. Obviously, developers want to avoid that from ever happening, hence why the testnet phase acts as a safeguard to prevent that. Essentially, all transactions are in demo mode during the testnet phase.
Additionally, addresses on the mainnet have different prefixes compared to addresses on the testnet.
Mainnet addresses generally begin with 1, 3, or bc1, while testnet addresses begin with 2, m, n, or tb1.
For example, you can find a list of prefixes that are used in reference to the Bitcoin blockchain on Wikipedia.
Also, tokens on the mainnet can’t be sent to the testnet, and vice versa. If you tried, the tokens would essentially be destroyed and unrecoverable.