Plain meaning
Start with the shortest useful explanation before going deeper.
The Anchor macro that defines the accounts struct for an instruction. Each field specifies an account with validation constraints. Account types include: `Account<'info, T>` (deserialized), `Signer<'info>` (must sign), `Program<'info, T>` (program reference), `SystemAccount<'info>`, and `UncheckedAccount<'info>` (no validation, use carefully).