POP stands for "Probability of Precipitation" and it refers to the percent chance that a specific location will receive measurable rain or snow.
When an area has an 80 percent POP of rain or snow in the morning, for example, it doesn't mean that there will be precipitation for 80 per cent of the morning, or that 80 per cent of the area will be covered in rain or snow. It simply means that if this exact same storm system went through 10 times, 8 out of 10 times it would bring you precipitation within that forecast period.