The manuscript does not reach the required quality standard of this journal
The answer you received indicates that your paper was a &quot;desk rejection&quot;. The editor did not send out your paper for review as she/he believes that your paper has no chance of getting accepted.
Journals accept some papers and reject others. To save the time of reviewers, editors of a journal often have a look at a paper and gauge its chance of acceptance before sending it out for a review. A few aspects that they look for are:

Is the grammar/spelling sufficiently good? It does not have to be perfect for the submitted version, but reasonably good. Papers for which the grammar obstructs the understandability of the content should be fixed first before doing a proper review.
Is the contribution as stated in the paper substantial enough for paper acceptance in this journal even it the reviewers find that the paper is 100% correct and publishable? If the claimed contribution is not perceived to be on par with the other published papers in the journal, the paper won't make it in, and hence there is - again - no reason to send this out for review.
Does the paper overall appear to have substantial flaws? For instance, are non-trivial claims not accompanied by proofs, does the paper structure follow the conventions of the field, etc.?
Does the appearance of the paper match one of a paper from an established author? Do the figures look roughly professional, is there a change in font in the middle of the text, etc.?

If your paper falls short on one of these aspects (and a few others that I most likely have forgotten), you are likely to get a desk rejection. Note that it ultimately saves you time, as you get the answer earlier in this way. However, the editor could have been a bit more precise w.r.t. what exactly was wrong. The normal course of action in such a case is to ask your advisor for advice if you have one.