What is a null hypothesis?
A null hypothesis is a default position that there is no effect or no relation between two groups, objects or whatever you might be researching. You then use data you collect to disprove this.
An example would be if you were comparing test scores between two groups, say blonde people and dark haired people. The default position (the null hypothesis) would be that there is no correlation between hair color and test scores. You then do a study on these two groups to try to disprove your null hypothesis.
Basically, your null hypothesis is the statement you're testing primality.