Plain meaning
Start with the shortest useful explanation before going deeper.
A persistent public Solana cluster intended for application development and testing, running the same software version as Mainnet Beta but with no real economic value. Devnet SOL can be freely airdropped via the CLI or faucet APIs, and the ledger may be reset periodically by Solana Labs. Developers use Devnet to test programs and integrations before deploying to Mainnet Beta.