FIDL (formerly known as mojom) is an IDL and encoding format used to describe interfaces to be used on top of zircon message pipes. They are the standard way applications talk to each other in Fuchsia.
FIDL includes libraries and tools for generating bindings from .fidl for supported languages. Currently, the supported languages include C, C++, Dart, Go, and Rust.
The language-fidl plugin provides syntax highlighting for FIDL files in Atom.