An array of floats (an n-dimensional vector) which represents some piece of data like a text or an image. Different embeddings can be more or less close to each other, and this closeness indicates similarity.
The magic is creating the embedding, embeddings themselves are nothing special.
[0] https://writings.stephenwolfram.com/2023/02/what-is-chatgpt-...
My current belief is that a hundred thousand+ dimensional latent space, as in SOTA LLMs, seems to have enough dimensions to reduce a large array of cognitive skills into a proximity search.