'if' and 'if not' in python, looping to search for a particular substring in a text file. what's the difference between the two? when to use which?

Total noob here.

Language: Python3

I am doing a beginners course on Coursera, Python for Everybody(specialization) And also following the free textbook provided by course instructor Dr. Charles Severance. Link to the textbook.

I am trying to use a for loop in Python to search through a text file. I want to search for a substring through the whole text file. I am trying to do in two different ways. Link to the text file I am using.

Sample one:

fhand = open('mbox-short.txt')

for lines in fhand :
    if lines.startswith('From:') :
        print(lines)

Sample two:

fhand = open('mbox-short.txt')

for lines in fhand :
    if not lines.startswith('From:') :
        continue
    print(lines)

The output is the same for both pieces of code:

From: stephen.marquard@uct.ac.za

From: louis@media.berkeley.edu

From: zqian@umich.edu

From: rjlowe@iupui.edu

From: zqian@umich.edu

From: rjlowe@iupui.edu

From: cwen@iupui.edu

From: cwen@iupui.edu

From: gsilver@umich.edu

From: gsilver@umich.edu

From: zqian@umich.edu

From: gsilver@umich.edu

From: wagnermr@iupui.edu

From: zqian@umich.edu

From: antranig@caret.cam.ac.uk

From: gopal.ramasammycook@gmail.com

From: david.horwitz@uct.ac.za

From: david.horwitz@uct.ac.za

From: david.horwitz@uct.ac.za

From: david.horwitz@uct.ac.za

From: stephen.marquard@uct.ac.za

From: louis@media.berkeley.edu

From: louis@media.berkeley.edu

From: ray@media.berkeley.edu

From: cwen@iupui.edu

From: cwen@iupui.edu

From: cwen@iupui.edu

Now what I don't understand is what's the difference in using if and if not in the above-mentioned code.

What goes on under-the-hood when I use them.

How are they different?

When to use which?

PS: if I have made some mistakes in posting this question then please let me know, I would be happy to comply.

Thank you very much for taking the time to read all this. Any help would be very much appreciated.