La Nina refers to the stronger trade winds than usual experienced across the Pacific Ocean. It is essentially a more exaggerated version of what occurs normally in the pacific.
The effects of La Nina are the opposite of El Nino.
e.g. instead of Australia facing droughts, there would be an increased risk of flooding
La Nina refers to the unusually cold sea temperatures found in the Eastern tropical Pacific. This is because the warm waters are pushed so close to Australia that it allows more upwelling to occur.