When you create a feature like a photo gallery, slideshow, blog or forum for Drupal, it involves installation of quite a few individual modules. In addition, each module often requires a lot of configuration.
Wouldn't it be nice if you could just create a feature just once and reuse it time and again, no matter how many modules and much configuration it needs?
That's the goal of the Features Module for Drupal. Create something once. Re-use it easily on as many sites as you wish.