If you have great powers of concentration, that means you're able to focus all your attention on the matter at hand. Concentration can also refer to something that's clustered together or to the density or strength of a solution.
A concentration of people means that there are many of them in one area. Your city may have a concentration of artists living in the warehouses by the river. A high concentration of a substance in a solution means that there's a lot of it relative to the volume: the Great Salt Lake has very few fish because of the high concentration of salt. To say that you have good concentration skills means that you pay attention well.