Variables in python can change throughout your code - so once you assign it a value, that doesn't mean it will keep that value throughout your code. You could for example just change the value of a variable in your code e.g.
bill = 0.2
bill = 'fish'
After the 1st line of code bill equals fish after the 3rd line of code it now equals fish.
However this probably isn't an example you'll see in practice a lot. What is more common is that you see variables that are additive. So in the example you posted meal starts off with 1 value then has something added to it. Another thing that can be tricky to get your head around is that meal appears on both sides of the equation e.g.
meal = meal + meal * tax
This is ok. Think of it as taking the original variable value, doing something with it and then assigning it to the original variable name. It's worth understanding this as it is a very common way of changing variables.