A null hypothesis (H0) is a stated assumption that there is no difference in parameters (mean, variance, DPMO) for two or more populations.
Classic definition from this site itself
i hope this makes things clear
in cases where we wanna find out if there exist an relation between y and x , null would be no there does not hence less the p value we take that relationship exists. similarily if we are to find out is there any difference between these two populations etc null is that there is no difference and higher the p value higher the probability that there actually exist no relation or chances of there existing a relation is less.
hope this helps
rgds
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