Transforms a flat list into a hierarchic collection

LinqExtensions.cs

    public static class LinqExtensions
    {
        public static IEnumerable<T> AsHierarchy<T>(this IEnumerable<T> collection,
            Func<T, T> parentSelector, Expression<Func<T, IEnumerable<T>>> childrenSelector, T root = default(T))
        {
            var items = collection.Where(x => parentSelector(x).Equals(root)).Safe();
            foreach (var item in items)
            {
                var childrenProperty = (childrenSelector.Body as MemberExpression).Member as PropertyInfo;
                childrenProperty.SetValue(item, collection.AsHierarchy(parentSelector, childrenSelector, item), null);
            }
            return items;
        }
    }

Hi.This snippet seems very interresting for me, but how to invoke it?
I dont understand the x=>x.Parent? Can you give an example to point me in the right direction?

public class Category 
{
public Int32 Id { get; set; }
public Int32 ParentId { get; set; }
public string Text { get; set; }
public IEnumerable<Category> Nodes { get; set; } = new List<Category>();
}
model.Categories.AsHierarchy(x => x.Parent, x => x.Nodes);