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.
Rokinon/Samyang lenses will work on your Canon body, but for a given value of "works". It will mount, you can focus it, you can set the aperture, you will have accurate stop-down metering, and you can take an image with it. But you cannot autofocus. You cannot have wide-open metering. You cannot shoot in modes other than M or Av, because the camera cannot adjust the aperture setting on the lens. There is no autofocus confirmation "beep"/green dot lighting up. And there is no lens EXIF information (focal length, max. aperture, lens name, aperture setting used, etc.)
Canon EOS lenses have a chip on them that communicates information from the lens to the camera and vice versa. The Samyang/Rokinon lenses do not have this chip or any electronic or mechanical communication with the camera bodies, which is why they're so cheap and come in any mount--they just have to physically fit the bayonet. You manually have to set the focus with the lens's focus ring (no autofocus, since that require body/lens communication), and you manually have to set the aperture with the lens's aperture ring.
There are 3rd party chips out there that you can glue onto manual or adapters that "fake" being a Canon chip. It doesn't let you autofocus or control the aperture from the camera body, since the mechanical linkages don't exist; but you can get autofocus confirmation and EXIF information from these chips. They're not necessary to shoot with the lens. And you do have to be very precise when gluing the chip onto the lens mount, since there's not a lot of leeway on contact placement for proper communication. And the chips you can buy vary on functionality (e.g., some chips allow you to capture the aperture setting you're using in EXIF, some don't; ditto storing autofocus microadjustment settings).