It is a way of quantifying the strength of the evidence against the null hypothesis and in favors of the alternative.

P-value quantifies how strongly the data favor HA over H0.

It can be said to be a conditional probability.

If H0 is true, then the probability of observing large sample mean will be low.

It is the least significant level at which you can accept the null hypothesis. It means there is a low probability of seeing a point estimate at least as extreme as the one we saw.

More simply, it is the probability of H0 to be true; if it is equal or higher than the significance level selected then we will fail to reject the H0 and if it is lower than we accept the alternative.