Suppose you are trying to get into the business of writing a blog. By itself the blog does not generate any revenue as it is usually free to readers. However, if you can make the blog popular, then you can attract advertisers. They can post advertisements on your blog. When readers click on their advertisements, you get a cut. This is an example of a business where there is no income from the main product and/or services. The main source of income is from the advertisements.
A less direct way of earning income from advertisers is product endorsement for bloggers. Basically you write a favorable review of the product on your blog and get paid for it. Whether that is ethical or not is another matter altogether. However if it is indicated that the blogger was paid by the company to review its products or services, then it is up to the readers to make their own judgement.