A while loop repeats while a condition is true. A for loop is different: it goes through a collection of things, one at a time, and runs its block once for each. A string is a collection of characters, so:

for letter in "cat":
    print(letter)

Each time round, the variable letter holds the next character: first "c", then "a", then "t". When there are no characters left, the loop ends. You choose the variable's name; letter, ch and char are all common.

Compare it with the while version, which has to manage the position itself:

word = "cat"
i = 0
while i < len(word):
    print(word[i])
    i = i + 1

The for loop can't forget to move on and can't run off the end. When you are going through every item of something, use for.