the for x in y statement takes iterable y and assigns a value from it to x per iteration (loop), so what happens is that it's reassigning chosen_char each loop to the next item from the sentence

(sum([x for x in sentence if x == chosen_char]) would be a quick one-liner, presuming one has downcased the sentence and other input/safety checks)

(e: this post was in response to your 2-liner comment in the code)