The term "repeater group" refers to a feature of the drag-and-drop editor. Repeater groups allow the inclusion of multiple rows of data in your content. The structure will repeat itself when multiple rows of data are found.
A customer places an order online to purchase 4 items. You have a triggered order confirmation setup in your account to include details of the order placed. The design of that confirmation email contains a repeater group to merge in line items from the order, like product description, product image, product quantity, and product price. The repeater group used will "repeat" itself 3 additional times to account for the 4 items purchased in that order.
How is it different than a cell group?
A repeater group IS a cell group. It simply behaves differently in that it can repeat itself where multiple rows of data exist. The repeat action is controlled by the segment the content is paired with in a campaign.
1. Repeater groups are functional only in data relationship of one to many (multiple records exist per email address) and available to professional license or above.