You may not be aware of an issue with an employee until you receive the notification from ACAS. Your employee (or former employee) can go to ACAS with a workplace issue and request assistance to resolve it. The employee (or former employee) could also have contacted ACAS as the first step to bringing an employment tribunal claim against you.
It is important to take legal advice when liaising with ACAS as, if they resolve the dispute, you will be entering into a legally binding agreement. It could be that an employee has been dismissed and they are claiming it was unfair. If ACAS resolves the dispute and you enter into a settlement agreement (this is known as a COT3), this agreement will be binding, and it is important that the employer’s interests are protected.