It's a measure of segregation. The higher the number, the more the group in question is concentrated in certain pockets.
For instance, here's a 2017 dissimilarity index for immigrants in the big three Canadian CMAs:
https://www150.statcan.gc.ca/n1/pub/.../tbl11-eng.htm
You can see that the cities of Montreal, Toronto and Vancouver are all similar, but Montreal's CMA has a much higher dissimilarity index because off-island suburbs had significantly less immigrant population than the city, whereas immigrants are more evenly spread throughout the suburbs of Vancouver and Toronto.