There are 5 anchor boxes, each with two dimensions. The first is the width and the second is the height. The dimensions are scaled in units of the grid cell size. Note that the whole point of anchor boxes is that they are “floating” and are used to find actual bounding boxes that have the same aspect ratio but which will have fixed coordinates relative to the grid cell that contains the centroid of the object in question.
In other words, the purpose of anchor boxes is to make it easier to find plausible bounding boxes for objects in the scene. The notebook and the lectures don’t really say enough about them, but they are learned a priori through a separate training process. There are some really epic threads on the forums about this and other YOLO related subjects, here’s one specific to the anchor boxes question as a place to start.
Hope this helps. If you want to dig deeper on this subject, you can also try reading the paper on YOLO2 (aka YOLO9000). Note that the anchor boxes were added in YOLO2 and were not part of the original YOLO paper.