Theory:

The physical quantities are measurable and have a numerical value (a number) and a unit of measurement.
Consider that you are buying 2 kilograms of fruits in a shop. Here, 2 is the numerical value and kilogram is the unit. Let us see about units now.
 
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.
Unit should be:
 
1. Well defined.
  
2. Easy to compare
  
3. Internationally accepted.
  
4. Independent of changing the physical conditions.
Example:
Centimeter is the unit for measuring length. We use this unit measurement to measure our height, our shirt's length 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, handspan or foot distance.
 
Therefore we need a system of  unit measurement suitable for all the people across the world. Essentially for this standard system of units to measure physical quantities were proposed.