Plain meaning
Start with the shortest useful explanation before going deeper.
A persistent bidirectional connection to a Solana RPC node's WebSocket endpoint (typically on port 8900) that allows clients to receive real-time push notifications when on-chain state changes, rather than polling via HTTP. The subscription model supports account changes, program account changes, slot updates, root changes, signature confirmations, and log emission, each returning a subscription ID used to later unsubscribe.