what is the difference between radd() and add( ) method of data frames in python?

Question

import pandas as pd
student = {\'unit test-1\':[5,6,8,3,10],\'unit Test-2\':[7,8,9,6,15]}
student1 = {\'unit test-1\':[3,3,6,6,8],\'unit Test-2\':[5,9,8,10,5]}
print(ds.