Perhaps you are new to WordPress or you are a site administrator and not the person who built the site. In any case, WEBphysiology has produced a video that will help you when it comes to adding a plugin to your site. Plugins are distinct pieces of code that can be added to a WordPress powered site to add functionality without having to re-invent the wheel. For some plugins that is all you need to do. For others there are options that may need to be defined to get everything set-up just the way you want them. Either way, the power and availability of plugins is just one of the great benefits of using WordPress as the publishing engine for any website.
This video will step you through the process of searching for and installing a WordPress plugin. The video also points out things to consider when deciding on a plugin you want to use.