What does the publisher mean by “Manuscripts […] are not returned to the authors”?
Back in the Olden Days, to submit a paper you would mail a hard copy to them. This notice says that they will not mail it back to you; they will only send you a letter with their decision. But nowadays, when we do things by the internet, this notice is largely an anachronism. However this journal does say it still can accept submissions also by ordinary mail.
Assuming you submitted your paper by Internet, I expect you will receive a decision on publication by email in due course.
As a mathematician, I always wait at least 6 months before I inquire about a paper I have submitted.