Possibility and probability

HI,
Can anyone out there tell me what does this phrase or saying mean...
"Possibility and probability are two different things"
OR
"Probability and possibility are two different things"
Thanks
Abhishek

It's not a phrase I'm familiar with. I think it may refer to planning for the future.
It's possible but not probable that there will be an earthquake tomorrow. I'm not taking any precautions.
It's possible and probable that Mount St Helens will erupt in the future. I'll be taking my holidays somewhere else!