It basically just means "hidden knowledge". It can be used pretty neutrally, one might refer to a largely unknown book about a particularly niche topic as "esoteric". Largely though, it carries a sort of mystical connotation, referring to knowledge that requires a great deal of metaphysical understanding to comprehend. Qabalah, alchemy, hermeticism, generally any topic concerned with uncovering the nature of reality through study of the qualities and patterns of the natural world.

Yoga is a broad spiritual practice, with the goal of using physical mastery of the body (including asana, the poses most people associate with "yoga") and mental mastery of the mind (meditation) to uncover hidden knowledge of the universe.

Naturally, a lot of vast writing on yoga can be described as "esoteric", including the practice of amaroli.