NUON stand for Nushell Object Notation and is intended to be the equivalent of JSON (JavaScript Object Notation) in the Nu language.
Files written in NUON are natively supported by Nushell, so a file like config.nuon with the following content
# config.nuon
{
foo: bar
}
will be opened and parsed automatically by Nushell. For example, this works without calling a parsing function (like from toml):
nu> open config.nuon
╭─────┬─────╮
│ foo │ bar │
╰─────┴─────╯
Note that in contrast to JSON, comments are allowed and that commas are not mandatory. NUON is in fact a superset of JSON as stated in the documentation. So every JSON is also valid NUON, but not the other way around.