What dual voltage ram means?

Ram voltage refers to the power consumed by ram module. DDR3 ram voltage used to be between 1.5-1.65v in the past, but now, most world leading ram memory manufacturers like  Crucial, Samsung or Hynix produce new DDR3L low voltage 1.35v modules, or, to be more specific, Dual voltage ram (1.35v / 1.5v).

Is dual voltage 1.35V/1.5V ram compatible with my original memory ?

If your originally installed memory is 1.5v and you adding dual voltage ram into another empty slot, then it will run at same voltage as your originally installed memory, which is 1.5v. In order to run at 1.35v, all installed modules must be 1.35v and your system motherboard must support DDR3L ( 1.35v low voltage ), to enable low voltage module run at 1.35v.

What is better, standard 1.5v or 1.35V/1.5V ?

In general, low voltage 1.35V is better because it consumes less power and produces less heat than 1.5v. However, you need to keep on mind, that your system motherboard must support DDR3L ( 1.35v low voltage ), to enable low voltage module run at 1.35v. If the system does not support DDR3L (low voltage 1.35v), then this module will run at 1.5v