Plain meaning
Start with the shortest useful explanation before going deeper.
An RPC method that scans all accounts owned by a given program and returns those matching optional memcmp (memory compare) and dataSize filters, making it the standard way to discover all state accounts for a protocol (e.g., all open order books, all user positions). GPA is resource-intensive because it performs a full account-store scan on the RPC node; many public endpoints rate-limit or disable it, which is why purpose-built indexers and services like Helius are used in production to serve GPA-equivalent queries at scale.