Consider a data frame df with 10 rows and index [ 'r1', 'r2', 'r3', 'row4', 'row5', 'row6', 'r7', 'r8', 'r9', 'row10']. What does the aggregate method shown in below code do?

g = df.groupby(df.index.str.len())

g.aggregate({'A':len, 'B':np.sum})