While DDR does stand for Double Data Rate and works as outlined by bharathbala2003, what is usually meant by Dual Mode is different.
Some motherboard chipsets can operate in such a way that when you have two DDR RAM sticks in two separate slots, they work together as a pair at a higher bandwidth in addition to increasing the amount of RAM.
So if your motherboard supports dual-channel mode, your system will be faster if you have, say, two 256MB modules instead of a single 512MB module. The level of overall increase in performance can be expected to be of the order of a few per cent.
Some Athlon XP motherboards such as those with nForce 2 chipsets support dual mode. Socket 754 A64 systems do not support it.