What Is Null Hypothesis?

Null hypothesis is a statistical term telling about the hypothesis of a given population parameter. It has a symbol H_O and stand for a plausible scenario given in order to explain some particular data. It is there too explain whether or not there is a link between two sets of variables or if two population parameters are different from each other. The null hypothesis is used when the researcher is generally unsure of the result. Depending on the situation the null hypothesis is either accepted or its is rejected. If the null hypothesis is not accepted than alternate hypothesis is accepted. The null hypothesis is used to test the significance of differences. Basically it tells that  the result would have been the same based on play of chance.
To understand what a null hypothesis is, consider the following example:
Suppose the two samples are male and female and you have to find out if their mean scores are different.
The null hypothesis would be that they were the same. Hence,
H₀ : μ₁ = μ₂