Start the explanation with how the metric units were derived from physical attributes in nature. The meter was defined originally as one ten-millionth of the distance between the North Pole and the equator. The liter is a cubic decimeter (decimeter=0.1 meter). The kilogram is the weight of water in a liter. So each unit is a function of a previously defined unit and some physical property in nature.
Introduce the different unit prefixes and their meaning. For example, “kilo-“ refers to “1,000 times,” “deca-“ refers to “10 times,” “deci-“ refers to “a tenth,” “centi-“ refers to “a hundredth,” and “milli-“ refers to “a thousandth.” A kilometer is 1,000 meters, the base unit of length measurement. A centimeter is a hundredth of a meter.
Compare the different metric units to units with which the audience may already be familiar, to help build their understanding. For example, a meter is a little more than a yard. An inch is 2.54 times the length of a centimeter. A kilometer is 0.62 miles. A kilogram is 2.20 pounds. A liter is about a quart.