shameless plug, usually included in a comment or post, when used by the person promoting something is meant as a #joking self deprecating manner of referring to oneself of promotion.
unless it is blatantly directed at someone else, it is meant to be almost apologetic.
Q: Why does everyone call their own comments "shameless plug"?
A: shameless because you have no shame in doing it? a lot of people are discreet when advertising but a shameless plug would be "Guys go buy my CD"
A: It sounds like it's a "shameless" promotion for something referring to the person speaking, "shameless plug" is a standard way of "apologizing" for promoting something related to the speaker.