By default, the gallery shortcode does not allow to limit the number of images displayed. This makes sense when you for example include all images attached to a post like this:
[gallery id="4"]
When this plugin is activated, you may use:
[gallery id="4" limit="3"]
and it will only display three images.
You can also use an offset parameter:
[gallery id="4" limit="3" offset="1"]
Will display at most three images, starting at the second.
Note that this plugin is a little bit of a hack.