Basically, the effect of the earth's rotation on the movement of air from high pressure areas to low pressure areas. Depending on whether it's the Northern or Southern hemisphere you will get right or left deflection. 
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let me futher explain it like this..Wind is caused by air flowing from high pressure to low pressure. Since the Earth is rotating, however, the air does not flow directly from high to low pressure, but it is deflected to the right (in the Northern Hemisphere;
; to the left in the Southern Hemisphere), so that the wind flows around the high and low pressure areas. This effect of the wind "feeling the Earth turn underneath it" is important for very large and long-lived pressure systems. For small, short-lived systems (such as in the cold outflow of a thunderstorm) 
the wind will flow directly from high pressure to low pressure. 
I hope it helps