This example seems a little odd, since it takes a data frame and mutates the x column to be equal (all entries of the column) to the sum of the second component

This example seems a little odd, since it takes a data frame and mutates the x column to be equal (all entries of the column) to the sum of the second component of the y column/vector to the 3rd component of the z column/vector. It is odd since they whole column turns into one value.