Python IF statement

'>>> # Measure some strings:
... words = ['cat', 'window', 'defenestrate']

for w in words:
... print(w, len(w))
...
cat 3
window 6
defenestrate 12'

I want to assume that I know what lead to the output but I do not fully understand the if statement. What is the use of the variable "w" ?
I mean , what is it doing ?