How to make popup text below heading on click?

Question

css

I have something like a menu. I need the text to float out from under it when I click on an item in the menu (smoothly). I made the text appear abruptly, but this does not suit me and in general it seems to me that I did everything somehow too clumsily. How can you make everything prettier and smoother?

Codepen: https://codepen.io/pen/?template=abWOQdX