please any one explain this INotifyPropertyChanged in MVVM

public  class NodeDetails:INotifyPropertyChanged 
    {
        private string _AvailableNodes;

        public string AvailableNodes
        {
            get { return _AvailableNodes; }
            set { _AvailableNodes = value; }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropetyChanged(string propertyname)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));

        } 
   
    }

i need explanation for below code

    public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropetyChanged(string propertyname)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));

        } 

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