An Influencer is an individual with the ability to influence potential buyers of a product or service by promoting or recommending the items on social media.
Normally they are people who publish regularly on social networks on a theme they have built a reputation on, either from their expertise or aquired knowlege, on a certain topic and that engage people, who see them as experts or role models, to follow them and their opinions on that topic.
A right influencer endorsement can help a brand build credibility and increase sales.