The term mainstream, in general, refers to all that is common. In marketing, the term refers to marketing aimed at the masses.
The term mainstream marketing indicates that the target audience of that marketing campaign is very large and broad. The products and services being offered can be used by almost everybody. Companies that cater to such broad audiences are called mainstream companies. Examples of mainstream companies include Google, Amazon, eBay, Procter and Gamble, and Coca Cola.
Going mainstream can be quite difficult, especially for startup companies. Many small businesses cater to niche markets. To be able to go mainstream, the company first has to make sure that they have a product offering that would appeal to people with very varied backgrounds.
For example, a company offering chili-flavored donuts alone will obviously have to stay content with selling to their niche market until they come up with other flavors and other products for their customers.
Another factor considered by companies that would like to go mainstream is their present level of success. If the company is not very competitive in its niche market, then chances are that it will not succeed in the mainstream market either. Third, it usually takes a lot of capital in order to transition into the mainstream market. However, despite the difficulties, if a company wants to turn into a billion dollar company it usually means that there is no choice but to go mainstream.