Plain meaning
Start with the shortest useful explanation before going deeper.
An HTTP callback mechanism used by Solana infrastructure providers like Helius to push real-time notifications to a developer-specified URL whenever a monitored address receives or sends a transaction, an account changes, or a specific program emits a log. Webhooks eliminate the need for client-side polling and are typically backed by Geyser-based indexers to ensure all on-chain events are delivered reliably with low latency.