The null hypothesis is the hypothesis that a factor or characteristic of a group is of no consequence. In other words, the null hypothesis is that a factor does not matter. More generally, the null hypothesis is a default position that the tester wishes to falsify on the basis of the data. The null hypothesis typically holds that something is not present, that there is no effect, or that there is no difference between treatment and control.
Many science experiments seek to disprove the null hypothesis as the first step towards proving that a meaningful (alternative) scientific hypothesis is true. Most medical studies, for example, that seek to prove the efficacy of a drug must first rule out the null hypothesis that taking the drug is no better than taking the current best treatment.