It means, normally a 550D comes with the older 18-55 IS lens, and now you will get the 18-55 IS Mk.II instead.
Note that optically, the two are identical. The only improvement is IS algorithm (the algorithm, not hardware)
Other than that it is cosmetic : the silver band is now painted black instead.
In terms of performance, there is no difference between the two, and the new IS algorithm provides a very slight improvement during a very specific type of shooting (panning shot)
In short, they are the same. The one with a silver band is called Mk I, the one without is called Mk II.
If you want any in-depth information, you can look through the very long and detailed reveiw here:
The sales is just trying to make you think you are getting a big offer - upgrading the lens for free. Well, now you know there is no difference between the two, it really isn't that big a deal at all. At most, the sales is just trying to fool you.