Low level programming skills in a low level language, C being the most popular. With memory safe languages like rust gaining more traction, C may eventually be a thing of the past, but imo, learning C will make you a better programmer in all languages because if you know C you know how everything works. Other than that, it could be something that uses a specific language like Arduino. Embedded programming is generally programming that interfaces with hardware directly, so really it's just knowing how to program, and then doing it in a specific area.

There's a good book called Code: the hidden language of computer hardware that goes in depth of computer architecture and build up from there.