Media Gallery fields in the Content Manager are used to store multiple images in one field in your collection. You can then easily display the images in a Gallery element on your site.
For example, let's say you have a real estate site. You created a database collection with all the properties you manage, and you want to include photos of the interior and exterior of each property. Using a Media Gallery field, you can upload all the photos you have to a single field in each item in your collection. You can provide a title and description for each photo right in the gallery.