When something is mainstream, it means it is gaining a lot of views to the point where everyone is doing it. The particular something is very popular that people are making big businesses out of that particular something. Sadly, that particular something out grows the popularity and it does not become mainstream anymore. Things that are not mainstream anymore becomes underground, meaning it is not trending but it is still popular and is possible to make something out of it. Something that is underground can potentially become mainstream if it is appropriate to the public’s eye.
When it comes down to contemporary design, being mainstream means that a product will most likely be re-invented in a unique way or help inspire other designers to design something similar of the mainstream product. An example would be rapper Snoop Doggy Dogg when he started evolving as a rapper under the tutelage of Dr. Dre and how his persona changed over the years.
The part where Steven Heller talks about copyright I kind of understand what he was saying. Copyright means protecting a person’s work. This means whatever content the person created cannot be used or published by anyone else without the consent of the originator. When it comes down to copyright, a lot of stuff on mainstream have a better chance of being recognized easier. The possibility of something being stolen is at a much lower risk, however it can still get an imitation.