Q: What does the status "Waiting for reviewer assignment" mean?
I submitted my revised manuscript to a mathematical journal since 12-04-2017. The status is "Waiting for reviewer assignment" since 24-4-2017. What does it mean? Have the reviewers started working on my manuscript yet?
This means that your manuscript has not yet been assigned to a reviewer. In this stage, the editor identifies a few reviewers for your paper and sends out email invitations to them. Once the required number of reviewers accept the review invitation, the status changes to "Under review." However, if one or more reviewers decline the review invitation, the editor has to look for other reviewers. While this may take some time, in your case, it's been nearly 4 months now. I think it would be a good idea to send an inquiry to the editor.
You will also find this ebook useful:
A guide to understanding journal statuses
Related reading:
Peer review process and editorial decision making at journals
This content belongs to the Journal submission & peer review Stage