Scoping is a concept common in many programming languages, it is a way to determine what is available for use in your code or not, and can be thought of as a tree of maps you can check for a specified name. Module-less languages like C have a single primary scope defined across the entire project (as all source files are simply concatenated together) while functions and blocks1 have their own nested scopes.
Module-based languages tend to have two scopes per module, the private module-level scope and the public export scope. Java allows only a single exported element per file (but multiple files per module, remember) declared
public while private elements are simply left undeclared. Node.js, Python, etc all have this two scope set for each module.
Alan continues that tradition here for ease of use, and like many of the languages does have a wholly-internal root scope that built-in types, functions, etc are defined in that all scopes eventually inherit from. The Built-ins section is documentation of this root scope and what it automatically brings to every module.