I have been coding (php, javascript, etc) for a few years. So I have some prior knowledge of the concepts of coding. I have had a pretty easy time with problem sets 0-3. However now that I am trying problem set 4, I feel like there are concepts and problems I have never even seen or heard of. I've watched every lecture, short and read all the links provided but I am still completely lost when reading through problem set 4. I'm sure I could hack my way through this problem set and questions, but I really want to understand what I'm doing and not just 'spray and pray.'

Did they raise the intensity level by a billion for problem set 4, or am I missing something obvious?