An influencer is someone with a strong social media presence, who has a large number of real human followers and the ability to "influence" the purchasing decisions of those followers.
If Linus from Linus Tech Tips says not to buy the latest AMD Radeon X921gtstx, a large number of people won't buy it. That's an influencer.
My understanding is that the metrics used to calculate ROI differ from client to client. It's more an art (or scam) than a science, with a ton of grey area. Whatever metric the client's boss is really into at the moment can be marketed hard and inflated using unverifiable data if needed. A lot of people are incentivised to tell the story that influencers provide value to brands. I believe that this is less and less true every day.
If Linus from Linus Tech Tips says not to buy the latest AMD Radeon X921gtstx, a large number of people won't buy it. That's an influencer.