Plain meaning
Start with the shortest useful explanation before going deeper.
Anchor's error handling system. `#[error_code]` defines an enum of custom errors (auto-numbered from 6000). `require!(condition, ErrorCode)` and `require_keys_eq!()` macros provide concise validation. Errors include the program name and error name in logs. Custom errors return in the transaction metadata and can be parsed by clients via the IDL.