Combinators in CSS

Question

everyone! I am quite new to HTML and CSS. I learned about 2 weeks ago. On my final website, I wanted to add a drop-down menu at the top. I want that when I hover over the 'store' tab, the 'shopping cart' tab slides down. I want to do it in only CSS. How do I reference one HTML element from another one from CSS code? What is the difference between > + ~ and other combinators. Sample code:

#store > #shoppingCart{}