Basically when you sell an investment property you can defer all taxes on that sale if you roll the money into another investment property. The deferred payment is not due until you sell the second property.
Say you make a $100K (estimated $20K capital gain) on investment property A, if you use the money to purchase investment B, and hold on to that property for 20 years, your deferred payment ($20K) remains what it would have been if paid at the time of sale of property A.