A unit is the name given to a specific quantity to enable consistent and coherent measurement of that quantity. The word unit represents 1 and so that allows the quantity to be scaled by that unit. We can represent larger or smaller values by using a different unit or by multiplying or dividing a unit often in 10s, 100s, 1000s etc with these factors being given a specific prefix.
A metre (abbreviated to m) is a unit of the quantity length. A 4 year old human child is about 1 metre (m) in length. We represent one thousandth of a metre (about the size of a grain of sugar) as 1 millimetre (mm) where milli m represents the number 0.001.
Here's an excellent video by Michael de Podesta describing how measurements and units are developed. https://www.youtube.com/watch?v=dguZLChkRV8
The term Concept:Measurement/quantity is often mistaken for unit and vice-versa.