I haven't looked into Leana specifically, but influencers are people on social media who have a specific topic they post about and make that their job (or at least try to monetize it in some way). Her thing seems to be fitness, so that would mean that all her posts are about fitness; pictures, videos, "stories" (short 15 second videos that are deleted after a day) and so on. For whatever reason people like to follow such content creators. They make their money either through ad revenue or through sponsorships/partnerships.
(Fun fact, I've gotten multiple "offers" on my music instagram with sub-30 followers by various weird and shady jewelry accounts, that wanted to send free jewelry to me in exchange for a few pictures or videos wearing them. Feels more like a multi level marketing scam than anything else though lol )