I've found this explanation of differences between dimensions vs measures from Tom Brown from "The Information Lab" very helpful. It could be learned even before starting Free Training Videos and in this case a lot of things from the start will be much easier to understand and follow.
Few words from the author of this article:
"Understanding the difference between the blue and green items in Tableau is (IMHO) the single most important piece of understanding necessary to make Tableau function well.
It it so fundamental, that I can honestly say that until I understood this, I was pretty much guessing how Tableau would react to my actions."