List is a mutable type, unlike strings

Dear members, 

I'm a complete novice, finding my way throught the Pycharm Education Course.

I just came accros the following remark that I don't understand :

    "You can add new items at the end of the list, by using the append() method and concatenation.
     Unlike strings, lists are a mutable type,
    i.e. it is possible to change their content using lst[index] = new_item "

I understand what is being said about information being added to a list.
But what I don't understand is the remark about a list being a MUTABLE type,
and a string NOT ....And in order to explain that statement, the example
is given saying " lst[index] = new_item ".

So saying ...
   My_list[3] = "The value at the 4th position"
   My_list[3] = "The value at the fourth position"
is being considered as a mutation,
but saying ...
   Variable_my_text_string = 'Some value'
   Variable_my_text_string = 'And now another value'
is NOT a mutation ?

I feel that I'm missing the point here..

Thx, 

Kr, 

Piet.vandeweghe@gmail.com