Any employee may at some time have trouble at work. It could be working conditions they are not happy with or the relationships with their manager. Some employees walk away and leave the business without you even knowing that they were troubled and this is a great shame.
Some will raise a complaint more formally by putting in writing their concerns. In an ideal world we would want them to articulate how they would like to improve the situation. Obviously they want the grievance to be heard, and if possible, resolved. It is also clearly in an organisation's interests to resolve problems before they can develop into major difficulties.
It is best to manage grievances sooner rather than later and if possible informally. However, organisations should have formal procedures in place, noted in your employee handbook and contracts, and the formal route may be your only option.
Problems they might want to raise could involve:
Your terms of employment.
Pay and working conditions.
Disagreements with co-workers.