What is harrowing and what is its purpose?

Harrowing is a shallow-depth secondary tillage activity for smoothing and pulverising the soil, as well as cutting weeds and mixing materials with the soil. Soil of the seedbeds are pulverized by harrowing. Harrowing is done to destroy grasses and seeds in the field , to  cut crop residues and mix them with top soil of the field, and to break the big clods and to make the field surface uniform and levelled. Harrowing is carried out when the moisture content of the clods are reduced.