basically this means you can store twice the data of a single layer dvd.
see crx1600`s link for more info but in simple terms it means that on the dvd disc itself there are two layers that can be burnt or read, each holding up to 4.5 gigs of data each.
when the top layer is full up the lazer re focuses through the top layer which is transparent, to burn or read the second layer which also holds another 4.5gigs.
you may notice this adjustment the lazer does as a slight pause when viewing a dual layer dvd movie. usually you dont notice this if the editing guys place this in a fade to back though. for data storage there will be no pauses.