HRO stands for Human Resource Outsourcing. HR is getting outsourced to third party providers who can bring in the benefits of knowing the domain.
HR as an activity, it comprises of a group of activities, which include payroll management, training, staffing, benefits administration, travel and expenses management, retirement and benefits planning, risk management, compensation consulting, etc. These activities are outsourced by which the client can concentrate on their core competency.
In the US context, HR outsourcing is a huge area. For instance, nearly about 29-30 per cent of the outsourcing space is HR.