It uses a technology called image re-targeting - algorithmically determine which areas of the image are important and then delete other areas retaining those which are important.
Liquid rescale GIMP plugin is available for Windows, Linux, and Mac OS X. However it is not included in the default set of plugins shipped with GIMP. So to start using the Liquid rescale plugin, you have to first install it separately.
Liquid rescale plugin can be installed in most Linux distributions using their respective package management tools.
If you are using Fedora Linux, you can install the Liquid rescale plugin using Yum as follows.
$ su -c 'yum install gimp-lqr-plugin'
After you have installed the plugin, you can access it via GIMP menu - Layer > "Liquid rescale..." as shown in the image below.
For further details on how to use the plugin, please visit the Liquid Rescale Plugin home page.