Social division means that the differences overlap on the basis of 'lower' caste, race, religion, class or ethnic group then he/she will be treated as inferior, will be discriminated against, and be subjected to harsh treatment. For ex - in the US earlier, the DIVISION existed between blacks and whites. The blacks were poor , homeless , discriminated against even the law was against them.
When different social differences overlap, one particular social difference gains in prominence. This leads to the creation of social division and tension.