Advertising is the practice of putting a message about a product in front of someone who might buy it, usually through an exchange of money between whoever is selling the product (or their representatives) and whoever owns the place where the message (called an "advertisement") will be placed.
Some common places that people encounter advertising are: television, newspapers, on large advertising-specific "billboards" along highways or on buildings in cities, or, increasingly, on the internet—especially on social media networks like Facebook or search engines like Google.
The main danger factor to consider with regard to advertising is that the people who create the advertisements have motives other than to faithfully provide useful information or to spread good ideas. Specifically, they want to convince their audience to buy something or vote for something. As such, they may in some cases mislead their audience intentionally—a practice that Facebook has inexplicably endorsed. Other times, they merely use bad ideas to sell their product. A good example might be: primarily using women actresses in television advertisements for cleaning products. In that example, the people who create the advertisements are broadcasting the idea that women are responsible for cleaning homes, which is a regressive and sexist idea. So, there are multiple ideological dangers at play.
On a more practical level, advertising can be dangerous in the sense that it can cause undue irritation. Television advertisements are often louder than the normal programming. Online advertisements can be slow to load or increase pageload times, resulting in content moving around on slower connections—in other instances, they can be so intrusive that it is difficult to access the primary content of the website when they're around. These are dangers to the audience (who may be annoyed), to the advertiser (who may receive the negative associations the audience forms as a result of their annoyance), and to the people who sold the advertising spot (whose audience perception may also be hurt).
Recently, advertising (especially online) has also contributed substantially to the destabilization of electoral politics and discourse on an international scale.
Because it can mislead, annoy, and incentivize otherwise unmotivated actions, we consider advertising to be just south of medium-dangerous.
