What is a Human Development Index?

I am writing a paper that asks us to take a side on if Puerto Rico becoming a US territory was a good idea or bad idea and if it helped Puerto Rico. We had to compare several statistics of Puerto Rico to the statistics of countries close to Puerto Rico, like Cuba and Haiti. I chose one statistic as an example and that was Puerto Rico's Human Development Index (HDI). I found that the rate was .95 (the same as USA's), while Cuba's was .838. Can anyone tell me what an HDI exactly is and if Puerto Rico's rate is better than Cuba's (i'm guessing it is)?