It is pretty straightforward: the first number represents the total amount of
bytes that were received from the site that is being saved locally (divide the
number by 1024 to get it in kilobytes). The second number is the average
transfer rate. If you divide the total bytes that were received by the average
transfer rate you should get the total elapsed time for the transfer (in
seconds).

Note that the "bytes received" part refers to all the files that were received
in order to save the offline version of the website regardless of type (this
means that image files, html/text files, scripts etc are counted in this sum).