One thousand millimeters is equal to 1 meter. The meter is the standard unit of length in the International System of Units, also known as the metric system. "Metre" is the standard spelling for all English-speaking countries except the United States. "Meter" is the accepted U.S. spelling.
While the definition of meter has changed over time, since 1983 the universally accepted International System of Units definition is "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."
The metric system is based on 10s, where each unit is 10 times larger than the previous one. A millimeter is equal to 0.001 of a meter since it's 1,000 times smaller than a meter.