Its like Safaris Reading List, you can save webpages for offline reading or safekeeping. Pocket is a bit more powerful than Reading List, it is available for more platforms and has additional features, like tags and search.
I use it and really quite like it personally. I like having an app that works on any of my OS X or iOS devices where I can save images or articles or websites and sort them using tags to look at later. The extension on Safari works well for me, and I like the option of being able to add tags to it at that very moment as well. Keeps all of the articles and such I want to hold on to nicely organized.