The SI unit for length is the metre

The SI unit for length is the metre. the symbol for the metre is m. short lengths are measured in centimetres or millimetres. long distances are measured in kilometres
the metre is used as the unit of length in many countries. How can we be sure that the metre used is the same everywhere
To ensure that it is the same everywhere we need one special metre everybody can refer to. this is called the standard metre. the original standard metre (kept in france) is made of a special metal bar and kept at a fixed temperature one metre is the distance between two marks on this bar other countries have a copy of this metre the accuracy of any other metre rule can be checked against these standard metres.
since 1983 the standard metre has been defined as the distance that light would travel in a vacuum in of a second. this length is more accurate than the standard metres made from metal