Any number that describes the result of a measurement, is a physical quantity: for example, weight and height, are physical quantities that describe a person. Some of these quantities are fundamental, meaning they can only be described by the way they are measured, referred to as operational definitions.

From 1889 until 1967, the fundamental unit of time was defined as a specific fraction of the average solar day, the average time taken for the sun to arrive at its highest point in the sky.

Today, a much more precise standard is used, based on the energy difference between the two lowest states of energy of a cesium atom. When bombarded by microwaves of a specific frequency, cesium atoms move from one of these states to the other: a second is defined as the time required for $\small \approx 9.192631770 \times 10^9$ cycles of this radiation:

Also See: A Brief Introduction to Time in Physics

In 1960, an atomic standard for meters was introduced, using the wavelength of the light emitted by excited atoms of kyrpton. From this, the speed of light in a vacuum was measured as $2.99792458 \times 10^8$, allowing the meter to be defined as the distance light travels in $\frac{1}{2.99792458 \times 10^8}$ second.

The standard unit of mass, the kilogram, is the mass of a cylinder of platinum-iridium alloy: an atomic standard would be better, but we cannot measure mass at the atomic scale with the necessary accuracy.

Once the fundamental units are defined, we can introduce smaller or larger units, usually related to the fundamental units in multiples of $10\; \text{or}\; \frac{1}{10}$, hence a kilometer is $1m \times 1000$, and a centimeter is $\frac{1}{100}$ of a meter. These values can also be expressed in exponential notation, i.e $10^3m = 1000$ and $10^{-2}m = 1cm$.