Writing a single authored research paper in a math journal: Is this a significant achievement?
Writing single-authored papers in math is extremely common, much more so than in other disciplines. It's not considered a significant achievement on its own in the context of applying for jobs.

Writing a paper before the beginning of a PhD is however certainly a big plus for you. But how much this will help depends on the journal: if you had the misfortune of publishing in a bad journal, I am sorry to say that this will actually hurt your application. It also depends on your field: I'm in pure math, but maybe in some areas in applied math it's expected of applicants to PhD positions to already have papers. Your question is a bit too general.