"Spidering" is the act of programmatically downloading entire web sites to a local hard disk.  Within Safari Tech Books Online, this activity involves the attempted downloading of entire books.  There are several commercial applications that "save" websites, such as WebWhacker, WebZip, HTTrack, and others.  Use of these applications constitutes spidering on Safari Tech Books Online.  Some advanced computer users will contract their own applications to perform the same downloading process.