Q:

About explainer badge

Quick question: Is the explainer badge awarded just once? I've been awarded this badge already but yesterday I did the same thing to earn this badge again but I wasn't awarded. Is there any specific criteria or restriction?

A:

Yes, you can get it only once. I believe this is true for all badges except where the description explicitly states it can be awarded multiple times. 

A:

All the workings of the sites that is common to all sites is usually explained on the main Meta, if not it's appropriate to ask there. There are 39 badges that can be awarded multiple times, 95 that are awarded once, and even one ("Not a Robot") that is awarded manually.
A few are site specific and some are not available on the Metas, only the main sites.
In addition, after checking if you were eligible, if you think you did not receive a badge that is due to you it's possible to ask there for someone to check. Some badges are awarded (almost) immediately while others are awarded after a scheduled job runs (sometimes).
FAQ: What are the badges I can earn on each site, and what are the exact criteria for earning each badge?

Explainer

bronze; awarded once
Edit 1 question within 12 hours of answering it
It is one of 20 badges that count towards the candidate score displayed for a moderator candidate in an election

