What are constants and literary constants?

Question

I am learning Python and I'm just confused with Literal constants and constants. What it is? For what purpose do we use them? What is their difference from a normal variable?

Many thanks.

Edit:

I am a true beginner. As a beginner, how can I say, I know nothing about the programming world. How would I not know what an expression is, and vice versa.

I learn python using the book "A byte of python", and somewhere in the book I came across a section that talks about literals and constants. I share the section:

5.2. Letter constants

An example of a literal constant is a number like 5, 1.23 or a string like "This is a string" or "This is a string!" ..

It is called a literal because it is literal - you use its meaning literally. The number 2 always represents and nothing more - this is a constant, because its value cannot be changed. Therefore, all of these are called literal constants.

Where he says: "It is called literal because it is literal, you use its meaning literally," I just do not get this part. What is the book trying to say that we use meaning literally? the indefinite point is that the number 2 is a constant, because the value cannot be changed. How is this possible? we can change it, for example:

stack = 2
stack = 3

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Answer 1

OP

- ; , , . REPL :

>>> 2
2
>>> 'hello'
'hello'

2 hello , , , (, , , ). :

stack = 2
stack = 3

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:

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True = False
if True:
    print 'Hey' 
else:
    print 'WAAAT!'

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: True False .. - Python 3.x, , True = False, SyntaxError: assignment to keyword.