The value for the difference of (š1āš2) should be replaced by the value of this difference under the null hypothesis.

Can somebody please explain what š1and š2stands for In the ex3? My understanding was that we are using time user spends on the page, as a null hypothesis but apparently this is not the case.

š1and š2 stand for the true average session duration for the control and variation populations (not samples, in that case x_bar is used) respectively. The null hypothesis is that there is NOT a difference between these values so the difference of (š1āš2) is equal to zero under the null hypothesis.