This index varies between zero and 1. Two communities with an index of zero have no species in common. Those with an index of 1 share all of their species. Simpson's index is a composite measure of diversity. A community with 5 species, all at roughly equal abundances, will have a higher Simpson's Index than a community with 5 species at higher unequal abundances.
Communities are typically very uneven, with few common and many rare species, but they can vary.
Communities with more even species distributions have flatter rank-abundance curves. See the figure below for examples of curves from communities with even and uneven species abundances. Before you can create a rank-abundance curve, you need to rank the species in your sample from most to least common. The number of individuals is log transformed to linearize the curve. By putting data from different communities on the same scale, log transformation also makes it easier to compare data between different communities.