@builtins returns a tree of the complete set of Origami built-in functions.
The built-in functions are already prefixed with @ signs to reduce the chance of a name collision, but the @builtins tree lets you further disambiguate between a local file or function of yours whose name happens to start with a @.
For example, if your project happens to contain a folder called @map, then using @map in a local formula will reference the local folder instead of the @map built-in function. To reference the built-in map, you can write @builtins/@map.