Theory:

The physical quantities are measurable and have a numerical value (a number) and a unit of measurement.
Consider that you are buying 5 kilograms of fruits in a shop. Here, 5 is the numerical value, and a kilogram is a unit. Now, let us discuss about the units.
 
Unit:
  • The standard term used for measuring a physical quantity is called unit.
  • It is defined as a specific magnitude of a physical quantity that has been adopted by law or convention.
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SI Unit icons
  
The unit should be:
 
1. Well defined
  
2. Easy to compare
  
3. Internationally accepted
  
4. Independent of changing the physical conditions.
Example:
A centimetre is a unit for measuring length. We use this unit measurement to measure our height, length of a shirt etc.,
Many of the ancient systems of measurement were based on the dimensions of the human body. But, the unit of measurement can vary from person to person and also from location to location.
 
For example: Hand span or Foot distance.
 
Therefore, we need a system of  unit measurement suitable for all people across the world. Essentially, the standard system of units were proposed to measure the physical quantities.