Question

what are the significance of p value?

Answer 1

The p-value is the probability of observing the result you obtained, or a more extreme result, given that the null hypothesis is true. So, given that the null is true (usually that there is "no effect"), you are inclined to believe that your results will deviate from the null value if this probability (the p-value) is small. We pre-specify the significance level because it allows us to design some sort of threshold of accepted vs. rejected values. But, of course this threshold (often to be 0.05) is considered the "norm", but you can you any significance level you desire.

So, when the probability of occurrence (the p-value) is smaller than the pre-specified significance level, then you reject the null hypothesis because you believe that this result is very unlikely to occur (because there is a very small probability that it would occur), and thus believe that there is evidence to believe that the alternative hypothesis is true.

You should note though that the p-value is a conditional probability where we assume that the null hypothesis is true. Thus, a p-value is really just a measure of the strength of evidence against the null. Also, the p-value is dependent on sample size. So, you are more likely to find cases where there is not enough evidence to reject the null hypothesis when you have small sample sizes.

Answer 2

you can use any significance level you desire*