Work content skills are ones that are needed for a specific job. They do not do you much good in terms of qualifying you for a job other than the one for which they are suited. Transferable skills, on the other hand, are more general skills that are related to a number of different jobs.
For example, there are certain work content skills that allow you to be a teacher but do not get you ready for anything else. These are things like learning a lot about your content area or learning how to write lesson plans. By contrast, other things that teachers do are transferable. This includes things like the ability to find information and to impart it to others in written or oral form. It includes things like the ability to make plans for complex projects (courses) that unfold over the course of a few months.