A Testnet is an alternative Bitcoin Blockchain that is mostly used by
developers to test their code, test edge case transactions and other
development related subjects, the testnet is currently
1412193 blocks long and
takes about 2 days to fully sync.
This caused some conversation on the bitcoin-dev mailing list calling for a reset of the testnet to make it sync faster and use less disk data, while some developers called for this, others actually called for a larger blockchain compared to the mainnet to find size-related errors. Developers like Jimmy Song and Johnson lau called for the existence of two testnets, one small and one large.