RAM runs in dual channel mode when you use 2 sticks of RAM.
And single channel mode when you use 1 stick of RAM.
The difference is the width of the 'road from the CPU to RAM', so to speak.
So in single channel mode the data width would be 64 BIT and in dual channel mode the data width would be 128 BIT. And for obvious reasons dual channel would be beneficial.
You can buy dual channel RAM 'pair' that has been matched to worked together in dual channel mode.
So if you need a specific amount of RAM, lets say 2GB, then get 2 X 1GB RAM so they'll run in dual channel mode. Buying 1 X 2GB RAM will give you your 2GB but it'll only run in single channel mode.