Centimeters
Short lengths are measured in centimeters, written cm. A finger is about 1 cm wide, and a pencil is about 15 cm long. The ruler here is 10 cm long: from 0 to 10 there are 10 spaces, each 1 cm.
A ruler 10 cm long, with the whole 10 cm marked.
A meter is 100 centimeters
Longer lengths, like a room or a door, are measured in meters, written m. Lay ten of those 10 cm rulers end to end. They make 10 × 10 = 100 cm, and 100 cm is 1 meter.
So 1 m = 100 cm. A big stride is about 1 m, and a door is about 2 m tall.
A strip 1 m long, cut into 10 equal pieces. Each piece is 10 cm, the length of the ruler above, so the 10 pieces make 10 × 10 = 100 cm.
A kilometer is 1000 meters
Long distances, like the way to school or from one town to the next, are measured in kilometers, written km. Kilo means a thousand, so a kilometer is a thousand meters: 1 km = 1000 m.
A 100 m race is run along a straight track. Ten of those tracks, one after another, make 1 km. Walking 1 km takes most people about a quarter of an hour.
The line counts meters from 0 to 1000. The end of it, 1000 m, is 1 km.
Millimeters
Look closely at a ruler, and each centimeter is cut into 10 smaller spaces. Each small space is a millimeter, written mm, so 1 cm = 10 mm. Millimeters are very small: a coin is only one or two millimeters thick.
The first centimeter is marked. The small marks cut it into 10 millimeters.
Choosing the unit
Pick the unit that fits the size of the thing. A pencil is measured in centimeters, a room in meters and a journey in kilometers.
The same length can be written in either unit, and a smaller unit always needs a bigger number, because it takes more small units to cover the same length. 3 m is 300 cm, not 30 cm, and 2 km is 2000 m, not 200 m. If a length gets a smaller number when you change to a smaller unit, something has gone wrong.