If at least one entry is selected, a "bookmark this page" button will display on all gallery pages. When clicking on this button, site visitors will see the icons that allows them to add the page they are on quickly to their favorite social bookmark service. You have to understand that displaying those icons does not automatically add links to your gallery: it just offers the visitor an easy way to bookmark your site. Displaying the social bookmarks section will not have a direct impact on your page rank, as the social bookmarks section is being written on the page using JavaScript (which search engine spiders will ignore). However, if users like your gallery and bookmark it using their favorite social bookmarking services, you will get back links from those social bookmark services which will increase your site's relevance in search engine catalogs. This being said: social bookmarks can be a legitimate tool to improve your search engine rank. As a disadvantage, the icons may distract visitors from the actual content of your site and lead them somewhere else. Therefore, you should consider carefully if you want to display social bookmark links and, if the majority of visitors of your page will understand and use the social bookmark icons. The social bookmark icons may be beneficial for gallery owners with a target audience that is widely familiar with social bookmarks. If you (as site admin) have no idea what those icons are for, you better leave them turned off (which is the default).
Consider carefully what social bookmark services you will display on your page - it's advisable not to display all that are available: this would only clutter the look of your page and disctract users too much. It would be confusing for users to actually find the icon of their favorite social bookmark site. There are many localized social bookmark services, so they are only usefull if your site visitors actually speak the language the bookmark service is providing.
To avoid spamming, most social bookmark services only allow bookmarks from registered users, so you can't expect that your site visitors will go through the registration process on all those services - they will probably only use the service they have already signed up for. That's why it's probably a good idea to only present a small number of those icons that are well-known in the community you catter for.
The social bookmark selector on the config page is using a select field where you can highlight multiple entries by holding down the [CTRL]-key on your keyboard (on PC clients). Use that key as well to unselect entries - if you just click on an entry without holding down the [Ctrl]-key, the item will be selected; re-clicking will not un-select it, so you will have to use the [Ctrl]-key if you want to disable the display of social bookmarks after having enabled it once.
This feature will only work if your theme contains the social bookmarks placeholder.