One of the things I've had most trouble explaining to folks learning R is the grammar of graphics. It's the theoretical underpinnings of the ggplot2 package, which is used to make all sorts of graphics. In general, I try to avoid being too theoretical in my courses and workshops, but the grammar of graphics is so key that you can't learn ggplot without understanding it.
In putting together my Fundamentals of R course, I made this video to help my students learn the grammar of graphics. I hope it might be helpful to you as you start your R journey.