Diversity is a politically correct euphemism for a divided society. In particular, it refers to mass immigration creating a divided society but may also refer to increased prominence of other minorities (such as sexual minorities) and reduced homogeneity in a society in general.
Such "diversity" is assumed to be something beneficial in itself which is highly dubious. Language diversity causes a need for translating which is unnecessary if everyone speaks the same language, religious and ethnic diversity causes religious and ethnic conflicts, cultural diversity creates misunderstandings, "racism" against immigrants causes the need for tax-payed "anti-racist" organization and "anti-racist" educations, the different groups will create lobby groups for their own self-interests, and so on.
Those who promote "diversity" often also promote race mixing which if universally practiced would actually eliminate the desired racial "diversity". In practice race mixing does create more "diversity" by creating new mixed-race groups but such and other effects of race mixing have many problems.
Research on ethnic heterogeneity has found that divided societies have numerous and severe problems.