Some CSS properties have inheritance. This means that if that property isn't specified for a css element, that element will inherit the value of that property from its parent. For example, a child element will default to being the same color as its parent. Some properties aren't inherited, and each element always gets its default value, no matter what its parent, such as the property "display".
You can use CSS to target specific elements on the page- a compound selector such as "header nav ul" looks for specifically an unordered list that's within a nav block that's within a header. This is useful, so you can have a different style for a ul that's elsewhere on the page. A multiple selector assigns the same propert to multiple elements- a selector of "p, nav, a" would assign the enclosed properties to all paragraphs, all navs, and all anchors. You can do a multiple selector with compound selectors, too.
IDs are only supposed to be used to identify one element on the page- each ID should be unique in best practices. Classes are meant to be repeated, the same class can be given to as many elements as you want.