Example

      int a,b;
        
        public void withThis(int a, int b)
        {
     //here this.a and this.b will refer to a,b of class not of this method.
            this.a = a;
            this.b = b;
        }
        public void withoutThis(int a, int b)
        {
            a = a;
            b = b;
        }
 
        private void btnThis_Click(object sender, EventArgs e)
        {
            a = 0;
            b = 0;
            withThis(5, 4);
            MessageBox.Show(a.ToString() + "\n" + b.ToString());        
        }

Here value of a and b is changed to the value of argument passed 

        private void btnWithoutThis_Click(object sender, EventArgs e)
        {
            a = 0;
            b = 0;
            withoutThis(5, 4);
            MessageBox.Show(a.ToString() + "\n" + b.ToString());
        }

Here value of a and b doesn’t changed to the value of argument passed