This is the lens focal length you will be using for the photo. You should always choose a focal length in the middle of your zoom's range so that you can zoom in/out slightly if required. The default focal length is 35mm.
Short focal lengths (less than 35mm) should be avoided as very small changes in the focal length lead to dramatic changes in the size and shape of the group. This is especially true when combined with shallow tilt angles (less than 35o).
You can change the default focal length in Settings (Pro version only).
IMPORTANT NOTE: If your camera does not use a full-frame 35mm size sensor then you must specify the 35mm equivalent focal length.