So answering the question: "What is influencer marketing?" I would advise to regard it as the investment first of all. It's the investment in brand name building. Though most young companies think that brand name is self-regulated and, what is worse, positive by default, it is not so. In reality, it is necessary to shape it day after day and invest in it. Influencers are the most reliable and predictable means of investment into a good and bright brand.
Influencer marketing is built on trust between influencers and their audience, and this trust is underpinned by the attention and time people are ready to spend communicating with the influencers they follow on Instagram. More than that, influencer marketing is correlated with the high level of customer engagement: an active interplay of users with the brand, their communication with the company through influencers.