Sometimes in chemistry we need to work with very small amounts or very large amounts. For example, if you were finding the thickness of a quarter, you might measure it in millimeters. If you were finding the length between one city and another you might measure it in kilometers. The base unit for length is the meter, and Milli- and kilo- are both prefixes that help describe lengths larger or smaller than the base unit.
You will want to memorize the metric prefixes to use when converting from one unit to another.
