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

[Larz60+]

test2(test1())

if you called test by itself, you would do something like:

df = test1()
test2(df)
# or 
xx = test1()
test2(xx)

so the last line is simply a shortcut, it states that test1 is to be executed and the results will be the argument to test2.
If test1 did not return a value, it would fail, try it.