This is a technical legal term used to describe intentional negative treatment of an employee (or deliberate failure to act) short of dismissal that has a significant negative physical or economic impact on them and that results, at least in part, from behaviour such as blowing the whistle on bad employer behaviour, or trying to exercise statutory employment rights, such as the right to take maternity leave.
The threat of negative action can also be a detriment, even if it doesn't actually happen because it deters you from exercising your statutory employment rights.
If you believe you have suffered a detriment at work, the first step usually is to raise the issue informally to see what can be done. If this is unsuccessful, you may want to use your employer’s formal grievance procedure. Every employer must have one.
If you think you have suffered a detriment for reasons relating to discrimination, consult Acas' guidance on asking your employer questions about suspected discrimination (PDF, 164KB).