A physical quantity is the property of an object that can be quantified. For instance, the physical quantities are the length of a rod or the mass of a body. Measurement is the act of comparing a physical quantity with its unit. The measurement result is the value of a physical quantity obtained by means of measurement. Measurement plays a vital role in our day-to-day lives. When we measure something, we quantify it. When you go to a grocery shop, you tell the shopkeeper to give a specific quantity of an item.
For example:
One kilogram of rice.
Two kilograms of wheat.
There are two major systems of units used in the world: SI units (also known as the metric system) and English units (also known as the customary or imperial system). English units were historically used in nations once ruled by the British Empire and are still widely used in the United States. Virtually every other country in the world now uses SI units as the standard; the metric system is also the standard system agreed upon by scientists and mathematicians. The acronym “SI” is derived from the French Système International.