esoteric means information that is only meant to be understood by a select few people, usually used in the context of occult/philosophical/religious knowledge.
Esotericism is a branch of western philosophical/religious/occult thought that involves the study of various eclectic hidden knowledges/schools of thought, like hermeticism, alchemy, freemasonry, theosophy, gnosticism, occultism, etc. The concept kind of formed around the enlightenment, which had a heavy masonic influence. Freemasons love larping and so in those days they larped as everything from satanists to ancient jews to egyptian alchemists to witches. It's like if you took isaac newton and mixed him with shadow the hedgehog, the result is aleister crowley and western esotericism.