Here -av is passed to rsync to run in verbose mode and print out the operations it is performing and to tell it to run in archive mode.
The files I want to copy are indicated by firstname.lastname@example.org:/* and mean that I am copying with the user pi from the Raspberry Pi with the IP address of 192.168.1.100. The final :/* means that all the files under the root of the filesystem will be copied.
The exclude flags specifically tell it to ignore the given list of directories. These are folders which do not contain useful files to back up and so are ignored.