Yaw is the horizontal rotation angle of the camera and part of the panotools Image positioning model.
If shooting a panorama you rotate the camera horizontally (around a vertical axis, TrY). You will treat one image as an anchor image[*] which is considered to have yaw = 0. The angle you rotated your camera relatively to this image is its Yaw value.
For a typical 360° panorama the yaw difference from one image to the next can be estimated by dividing 360 by the number of images.