What does 1: in Canon Zoom Lens EF-S 18-55mm 1:3.5-5.6 IS II mean?

It means exactly the same as f/. 1: is just an older syntax for denoting a ratio. You might also occasionally see 1/. Since the f-stop is essentially a ratio of focal length to entrance pupil diameter (the apparent size of the aperture when looking into the front of the lens), one of these notations is used to indicate, in this case, that at the 18mm end, your apparent entrance pupil size is 18/3.5 = 5.14mm, and at the 55mm end it’s 55/5.6 = 9.82mm.