In more detail
TOML stands for Tom's Obvious, Minimal Language. A file is a list of key = value pairs grouped into [tables], with strings, integers, floats, booleans, dates and times, arrays and inline tables as value types. Comments start with #, whitespace does not change the meaning, and defining a key twice is an error. Version 1.0 was released in 2021 and files are UTF-8 text.
Why it matters
Python packaging (pyproject.toml) and Rust (Cargo.toml) use TOML, and many tools adopted it because it avoids the pitfalls of YAML while allowing comments, which JSON does not. It is best for hand-edited settings, less so for deeply nested data. Convert to and from JSON with the TOML to JSON Converter and the JSON to TOML Converter.
Example
[server]
host = "localhost"
port = 8080
This parses to {"server": {"host": "localhost", "port": 8080}}.