Repeaters can be used two ways. First, they can be used purely for formatting. Any text or images you apply to an item in the editor will display in the editor, in preview, and live.
Second, elements in the repeater can be connected to a dataset. In that case, the dataset content will be used for preview and live display, but the data shown in the editor will be continue to be whatever data was applied via the editor, just as though it weren’t connected to a dataset at all.