Sometimes an employer has some policy or practice that appears to be fair to everyone but, in the real world, has negative consequences for certain employees only. If the employees who suffer are members of a protected class, then the policy is said to have a disparate impact and it is illegal.
For example, if a clothing store has a policy that they only hire employees who are over 6 feet tall, then this would have a negative impact on women because they are less likely to be 6 feet tall than men. or other protected classes getting jobs. If the policy has no legitimate business purpose, then it is employment discrimination.