I got the following comment from a reputed math journal. What does it mean?
My educated guess is that the reviewer is a native speaker of German and mistranslated some terms into English:


The German word Übung translates both to practice and to exercise.
In German universities, höhere Mathematik (literally: higher mathematics) was used as a label for mathematics courses for students of other scientific fields or engineering. At times, this only referred to the calculus courses (and not the linear-algebra courses). In particular, these courses usually focus on applying mathematical concepts and do not compare to courses for students of mathematics. (I wouldn’t be surprised if you can still find this label in use somewhere.)


If my guess is correct, the reviewers comment means something along the lines of:


  This manuscript is just a (demonstrative) exercise in undergraduate mathematics and therefore not suitable for publication in a high-level journal.


So, your manuscript was rejected because it allegedly does not feature any new mathematics.