The working memory is space in ones mind for storing things that you're actively reasoning about. When I'm cooking a large meal, I'm trying to keep track of all the dishes that are actively cooking or that I'm preparing. I'm also keeping track of timing for when to start new dishes or pull dishes that are done cooking. Those dishes that I'm tracking in my mind might be a part of my working memory. In some cases I can choose to leave some dishes out of my working memory, like if I haven't even started preparing them yet, or if I've set a timer to remind me when to pull them.
The theory use to be that a person could hold up to seven items in their working memory, plus or minus two. Those tests were conducted with simple objects, like numbers, that the person was expected to hold onto for a short time. Now they believe that the actual number is more like three or four objects.
See Sense of Style.