Iteration_variable

Hello

I have a quick question. What does for iteration_variable in content_ratings:
content_ratings[iteration_variable] /= total_number_of_apps do? Can someone explain me what the code is trying to say?

What does the content_ratings[iteration_variable] do? How does having it in brackets help. Is it like appending? Does the code in essence means that for every variable that repeats itself in the content_ratings, that the content_ratings should increase by that number (and divide itself by the total number of apps)

So for instance if the content_rating is ‘4+’ then everytime there is a rating associated with ‘4+’ then the iteration variable and then divide itself till its gone through the entire dataset? Please let me know!