Obviously this is an extremely subtle meme so I will try to break it down for you.

Black people (referred to in the image as niggers) have been found through psychometric testing to have lower than average intelligence. There is an oft quoted cliche that Polish people are also of lower intelligence and while the data doesn't bear this out, the meme drolly juxtaposes this with the low IQ scores of Blacks for our entertainment.

Please let me know if any further clarification is required.