If something is described as embryonic, it's just starting to develop or come together. An "embryo" is a person or animal that is still growing in the womb or egg, and embryonic means "like an embryo."
The idea for Facebook was still embryonic when its creators agreed to be partners. At that early stage, none of them could have imagined what an enormous company it would become. Scientists working on embryonic stem-cell research are working on stem cells from embryos, which have special properties not found in any other cells. Our relationship was still embryonic when you suggested we move in together, but for me, the first date is way too early to think about such things.