The p value is a statistical measure that indicates whether or not an effect is statistically significant.
For example, if a study comparing two treatments found that one seems more effective than the other, the p value is the probability of obtaining these results by chance. By convention, if the p value is below 0.05 (that is, there is less than a 5% probability that the results occurred by chance) it is considered that there probably is a real difference between treatments. If the p value is 0.001 or less (less than a 1% probability that the results occurred by chance), the result is seen as highly significant.
If the p value shows that there is likely to be a difference between treatments, the confidence interval describes how big the difference in effect might be.National Institute for Health and Clinical Excellence