Natural units are physical units of measurement based purely on universal physical constants. Well-known examples include the speed of light c, the electric charge e, and the gravitational constant G.
In an ideal mathematical description of our universe, all of these would be expressed simply as “1.” In practice, this is not often feasible as knowledge of the units of measurement, e.g. kilograms, meters, etc., are useful in mathematical formulations.
A reasonable analogy – though imprecise one – is the Unit Circle, which is a circle of radius “1.”
Each Natural Unit has a “metric value,” which defines the unit in more common terms. For example, the natural unit c for the speed of light has a metric value of 299,792,458 meters per second.