What do memory, storage, and calldata mean in Solidity?

storage is permanent and lives on-chain beyond the function; memory is temporary and modifiable, existing only during the call; calldata is temporary and read-only.

You specify a data location only for reference types — arrays, structs, mappings (and strings) — while value types like uint256 are handled automatically. Use calldata when you won't modify an argument, memory when you will.