Basically it means the memory has a total bandwidth of 20 Gigabytes per second. Memory bandwidth is the rate that data can be read from or written to the memory chips themselves.
In reality the max bandwidth is rarely achieved since other bottlenecks exist in systems.
The announcement is somewhat misleading as well. One DVD stores 4.7Gigabytes , and the memory bandwidth is measured in gigabits. 20 gigabits = 2.5 gigabytes (20/8 bits per byte).