Reverse discrimination is claimed whenever someone who is viewed as being the majority feels they were overlooked simply because they are not a member of a "group" which is considered a (minority).
An example might be a government contract goes to a woman owned or black/Hispanic owned company instead of a white male owned company which may have been in business longer and has a stellar reputation. The U.S. government guarantees a certain percentage.
Many folks also consider gender and race quotas to be unfair on jobs as well. The fire department for example may have 100 positions and (have slotted 10 of them for women or non-white males). Even if the 91st person is a white male with a higher test score he will not be allowed to join the force.
From the government's point of view they are trying to encourage diversity and make up for many years of sanctioned or unchecked discrimination towards women and minorities. The overall belief is a person in the (majority) will have a lot more other options. Nevertheless this person who didn't get the job or contract views it as reverse discrimination.