Va. Code 20-124.3 describes the best interest of the child factors a court must consider when deciding custody and visitation. Part of the second factor is a parent's mental condition. That is what the psychological evaluation seeks to establish.
You should contact the evaluator's office before your appointment and ask what will happen. Most likely, the evaluator will administer some standardized tests. There will likely be an interview with the evaluator. You will provide some life history to aid the evaluation. There may be more than one meeting required.
The evaluator will prepare a report. The report should be presented to the judge as some point to be considered with other relevant evidence in deciding custody.