This is what I've always considered to be the definition of promotional content (my wording):
One or more posts that focus on some product, organization, company, etc. that the author is connected with in some way to an extent that is inappropriate in the given context.
Here's an example of what might be construed as promotional content. It is an answer to the question of "How long will a star with a mass of ten solar masses stay on the main sequence for?"
I wrote a program for this in Python a while back that takes various factors into account to solve this problem. You can find it on my website, www.[redacted].com. It's free. Anyway, I plugged in your mass (ten solar masses) and my program output [X] million years. Cool, huh?
Besides being short, this answer unnecessarily focuses on the product being mentioned, in this case, a software program. You could easily just mention the program in a few words and focus the rest of the answer on the result (or you could quote the vast amount of scientific literature out there on stellar lifetimes).
If a user writes a bunch of this sort of answer, whether they be about the same product or not, they might be suspended for posting overly promotional content.
See also Limits for self-promotion in answers and Defining the limits of self-promotion.