EMCD, or electron momentum density, is a way to understand how electrons move around inside of materials like metals or crystals. Imagine you have a bag of marbles, and you shake it really hard. The marbles will move around inside the bag, bouncing off of each other and the sides of the bag. Now, imagine that the marbles are electrons, and the bag is a material like a metal. That's kind of what electron momentum density is like.
Scientists use fancy machines called electron microscopes to look at the electrons inside of materials. They can track how the electrons move and where they are located. By studying the electron momentum density, scientists can learn more about how materials behave and how they might be useful in different applications, like making stronger metals or developing new electronics. So, basically, EMCD helps scientists understand how tiny things called electrons move around inside big things called materials, and that can help make our world a better place.