What you get using the max function is the maximum value between two arguments.
For example, when I write this formula max ( green ; rededge ) I get values from 0.17-0.27.
If I display only the green band I get values from 0.04 to 0.09
If I display only the red edge I get values from 0.17 to 0.27
So the max of these two bands (green and red edge) will be the values from the red edge.
For information: Generate a vegetation index