When webpages are shared on Facebook and Twitter, there are three main elements of the share: the share image, share title, and share description.
By default, when your site's homepage is shared on social media, the share image is your organization's logo, or if that image isn't large enough, another image from the homepage (usually a carousel image) is used. The share title is your site's title and the share description is the site explanation shown on the homepage (to the right of the featured campaigns list).
However, if you'd like to override these defaults, you can do so by going to the admin homepage > Settings > Social Media tab. From here you can update the share image, title, and description.
Note: the site description will also be used by search engines when the site is included in search results.