This was a huge book, and in all honesty I only skimmed it. Wolfram talks about the power of building simple systems. Systems so basic that they range from 1 to 256 "rules" along the lines of "when these two things are together then do this." Even with these very basic rules (especially in computer terms) he was able to build highly complex layouts, patterns and systems. He continues by identifying how this is the way that nature generally works: a very basic "rule" set leads to very complex systems (think of weather patterns, the inner workings of a star, etc).
tidbit: twisted rope (the form of rope we generally use today) has been around since at least 3000 BC. Crazy fact to find in a math book.