4) Modified Duration
As explained in the second point, bond prices and interest rates are inversely related. This means, if there is a rise in interest rates then there is a fall in the price of the bond. If there is a fall in interest rates, then the price of bond will rise.
MD is simply the change in the value a debt security in response to the change in interest rates. So let's say the MD of the bond is 4.50. What this indicates is that the price of the bond will increase to 4.50 with a 1% (100 basis point, or bps) increase in interest rates.
This provides a fair indication of a bond's sensitivity to a change in interest rates. The higher the duration, the more volatility the bond exhibits with a change in interest rates.
Since modified duration of a portfolio takes into account all the debt instruments, it will change with regard to the composition of the portfolio.