get,set and value keyword in c#.net

What is value keyword here and how is it assigning the value to _num? I'm pretty confused, please give the description for the following code.

    private int _num;
    public int num
    { 
        get 
        {
            return _num;
        }
        set 
        {
            _num=value;
        }
    }

    public void button1_click(object sender,EventArgs e)
    {
        num = numericupdown.Value;
    }