If you own bonds with long duration you are also long convexity. In essence this means bond prices go up faster than they go down for a similar change in yield.
If you take the current benchmark 30y US treasury it has a coupon of 2,375%, a yield of 2,30% and a price of 101,62
If the yield goes up to 3,3% the price of this bond is 82,5
If the yield goes down to 1,3% the price of this bond is 126,6
When yields become really low this effect takes off big time. If the yield goes to zero the price goes up to 171,2.