This is the basics of the basics of the most basic concepts of graphical design. It tells you what the basics are, but only goes into the very definition of those words, not how to use it or what it actually means to use them in design. "Contrast simply means that one item is different from another.". It doesn't explain the differences between weight contrasts, color contrasts, shade contrasts, font contrasts, size contrast and so on (as an example).
Hence anyone who actually spend credits on unlocking this will have a huge let-down.