This command downloads a file:
wget "http://www.voipmonitor.org/download-gui?version=latest&major=5&festry"
If you just run this command on its own, the name of the file that will be downloaded will be download-gui?version=latest&major=5&festry", because that's what wget sees in the URL.
The -O option allows you to rename the file you're downloading. So -O w.tar.gz renames the file to w.tar.gz; so basically after the download finishes, you'll find a file called w.tar.gz, that's the file you downloaded.
The next command is:
tar xzf w.tar.gz
Which extracts w.tar.gz. It will extract a folder called voipmonitor-gui-6.0-SVN1062 and a file called voipmonitor-5.0-SVN-onlyforbackwardcompatibility.
The next command:
mv voipmonitor-gui*/* ./
Will move everything from under the folder voipmonitor-gui-6.0-SVN1062 to the current directory. So ./ means "current directory".
Example:
If you've downloaded the file to your Downloads folder, this will be your folder structure after extracting:
Downloads/
voipmonitor-gui-6.0-SVN1062/
[lots of files]
voipmonitor-5.0-SVN-onlyforbackwardcompatibility
After the move command, you'll move [lots of files] to Downloads.
Downloads/
[lots of files]
voipmonitor-gui-6.0-SVN1062/
empty
voipmonitor-5.0-SVN-onlyforbackwardcompatibility