Notes to yourself in the program
The hash mark, why it exists, and the trick of switching a line off.
Code is read far more often than it is written, and usually by the person who wrote it, several months later, with no memory of what they were thinking at the time. A note left in the program for that person is worth a surprising amount.
In Python you leave a note with a hash mark, #. Everything from the hash to the end of that line is ignored completely: Python sees the hash and skips straight to the next line. A note like this is called a comment.
Delete the # at the start of the first line and run it again. Python will try to obey the note and stop, because a note is not an instruction.
A comment can also sit at the end of a line of code, once the instruction on that line has finished.
Change both comments to say something else and run it again. Nothing about the output changes: comments are for people, and the machine never sees them.
Switching a line off
The second use of a hash mark is temporary. Putting one in front of a line of working code turns that line into a note, so it stops running, and taking the hash away brings it back. This is how you find out which line is causing trouble, by removing them one at a time until the trouble goes away.
Take the # off the middle line and run it again. Then put a # in front of the first line instead.
Move the # to the very front of the line, outside the quotation marks, and run it again. Nothing at all comes out, because the whole line is now a note.
A comment that repeats the code is noise. print(3 * 250) # multiplies 3 by 250 tells the reader nothing the code did not already say. The comments worth writing say why: why pence rather than pounds, why 250 rather than 300, why this line is here at all.
Does a comment slow a program down?
Not in any way you could measure. Comments are thrown away before the instructions are carried out, so nothing about them reaches the part of the machine doing the work. Write as many as are useful.