As the title puts it, what is the real meaning of this stage of a peer reviewing process and how are the odds for having a submitted paper accepted change if it's accepted for this stage? (Does it mean anything other than the editors decided not to reject it from the start?)
"First look" is a stage in the ScholarOne Manuscripts (S1M) system used by many journals. The First look stage means that your manuscript, after an "Accept" decision is scrutinized to see if it adheres to journal standards (a.k. Instruction for authors) and that figures are of sufficient quality (resolution for bitmaps). The journal editor(s) handle(s) this stage. Your paper may be returned to you to fix specific formatting problems BUT NOT changing the content. See S1M pdf
So "First look" is not a general name for a stage but is specific to S1M. Other systems or journals may of course be using such a term for other purposes but it is not a general publishing term. The stage it represent will, however be found in all journals.
I actually tried to figure some of this out recently as I've been working to get my first article published. I will try to elaborate on the little bit that I know about the review process and maybe others can chime in and let me know if I more or less have the process right.
First, you submit your article to the journal. It is then received and submitted to one of the associate editors I believe. I don't know how carefully they read it in this stage, but they make a decision whether to "desk reject" or send it out to the reviewers. If they desk reject your paper then no one else at the journal gets a real look at it. (I think this is what the accepted for first look means; that your paper is not desk rejected)
If they decide that it is worth having someone take a look at it then they inform you that it is accepted for a first look. At that stage they send it out to several other reviewers. For the papers I've submitted it has usually been 2 or 3 reviewers. They take somewhere between 3-9 months to read the paper carefully and provide good feedback. They then submit a recommendation for what should be done with the paper either 1) accept, 2) reject, or 3) revise and resubmit. If they accept or reject then the outcome is relatively self-explanatory. If they ask you to revise and resubmit, it means that they thought it was pretty good, but that they had some issues with the paper that would need to be cleared up prior to accepting the paper for publishing. You can make the revisions that they ask and then resubmit the paper and you once again get one of the 3 decisions.
I attached the guideline of S1M system, saying that " MANUSCRIPTS ACCEPTED FOR FIRST LOOK If your paper is accepted but minor updates are required before the final files can be sent to production you will be notified of this and will find the paper listed under "Manuscripts Accepted for First Look". Click the link and then click "submit updated manuscript". Further instructions on how to upload your final files can be found on the screen that follows. "
That means the MS is accepted with minor revision without peer review.
The workflow in each journal and conference might be very roughly comparable (because they all draw on the same mass of experience in the job, because people overlap/move from one to the other, because they pilfer ideas they see in the competition), but nothing suggests they will use the same terms to describe substeps in the process.
The only way to find out what exactly some uncommon term means to that journal is to ask them. Sure, you might chance upon someone who knows their terminology in detail here, but you won't recognize them if so either.
This means that the paper is sent out to reviewers and the chances of acceptance are at the mercy of the reviewers. If the reviewers find it good enough , it will be accepted(usually pending additional experiments). How the actual odds change depends on the journal but I'm not sure if most journals even publish these statistics in detail. I would sit back and wait for reviewer comments as you've done what you can. Congrats on the paper.