Passing data between functions, got it working, but need clarification please

[digitalmatic7]

I made a basic script that passes a dataframe between 2 functions.

I understand how it works, except for the meaning of the last line.

test2(test1())

Does this mean that all the code from test1 is merged within test2?

I get that it's calling both functions, but I don't fully understand why test1 has to be inside test2, and what exactly is happening at run time.

import pandas as pd


'''
test1 function creates the dataframe
'''


def test1():
    list1 = ["item1", "item2", "item3", "item4", "item5", "item6",
             "item7", "item8", "item9", "item10", "item11", "item12"]

    df = pd.DataFrame(list1, columns=['Col 0:'])  # create new dataframe from list1
    df.insert(1, 'Col 1:', "")  # insert new column
    df.insert(2, 'Col 2:', "")  # insert new column

    return df  # pass the dataframe outside the function


'''
test2 function calls the dataframe from test1() makes a change to the cell data and prints df 
'''


def test2(df):  # call df which was passed from test1()

    df.iloc[5, 2] = "NEW CELL DATA"

    print(df)


test2(test1())