Diversity measures how many different species you have (richness) and how evenly they are distributed (evenness). In general, a more diverse microbiome is associated with a more stable, resilient ecosystem that recovers better from disturbances such as antibiotic treatment. Low diversity is more commonly seen after antibiotics, with low-fibre diets and in certain diseases.
This is an association, not a diagnosis: some healthy people have moderate diversity. The most useful approach is to see how you compare with the reference population and how your diversity changes when you change your habits.