You won’t get far in web dev without understanding CSS selectors, and they aren’t hard at all!
Play with the code! https://codepen.io/anon/pen/BGeKVM
View the whole Web Development Tutorials for Beginners playlist! https://www.youtube.com/playlist?list=PLoYCgNOIyGABDU532eesybur5HPBVfC1G
Using CSS selectors are the bread and butter of HTML and CSS coding. You have to think about what elements on your page you want to target and how to write rules that cleanly target and apply css styling to those elements.
2 good rules you always want to follow: don’t make rules more specific than they have to be (but also don’t make them overly generic like “target every div on a page”), and also don’t repeat yourself (DRY).
Also watch: “Responsive Design Tutorial – Tips for making web sites look great on any device”