I want to buy the Rokinon 8mm f3.5 Fisheye lens, but I saw in one review that is not recognisable for Canon because there is no a chip which is necessary. What is that exactly? Will I have problems with my camera?
It is a manual focus lens, that will communicate essentially nothing to the Canon body.
If you are using a Canon OEM lens for example, even when you manually focus the lens, it will audibly and visually confirm that focus was achived at a certain focus point. With the Rokinon you will not have that feature.
On a fisheye lens, the depth of field is very high. You can simply use the distance scales on the body of the lens to set your focus and not worry about it all that much. For the price especially, you are getting a fairly high quality lens and autofocus is not typically necessary.
It is important to note that versions of this lens do exist that have the focus confirmation chip built in. I'm not sure if that is what you are looking at or not. It is usually made pretty clear by noting "with focus confirmation" or similar on websites that sell this item.