This is a title people give to you, not a registered profession. Linus, from Linux Tech Tips is an influencer insofar as he influences people.

While a lot of kids saying “I want to be famous” are a little cringe, if we want to be more charitable we should understand people who say ‘I want to be an influencer’ simply as ‘I want to do well at this job’, which isn’t a bad thing, unless the job is bad.