Basically, `mu` is the difference between `mean(sample_x)` and `mean(sample_y)`. It's usually written as Greek letter mu and indicates the difference between the mean values of each sample respectively. In the simplest case (and which is by far the most common), we test for `mu==0` to see if two samples have a same mean value. However, the 2-sample T-Test is constructed to test for `mu` of arbitrary value.