He means that the default transformation without styling just displays the images in a 1 column vertical format.
It's not the greatest default transformation, and it also doesn't help that the transformation selection in the widget properties isn't there by default. (It can be added in the CMSSiteManager site)
As for how to get the images to display in a columned setup here one possibility once you've enabled transformation selection for the widget (or changed the default):
Header and footer transformations would have <table> and </table>
<%# ((DisplayIndex % [column size]) + "").Equals("0") ? "<tr>" : "" %> <-- index starts at 0 and values of index MOD column size will range from 0,1,2... column size-1
<td><a href="<%# HTMLHelper.HTMLEncode(MediaLibraryFunctions.GetMediaFileUrl(Eval("FileLibraryID") ,Eval("FilePath"), Eval("FileGUID"), Eval("FileName"), GetDataControlValue<bool>("UseSecureLinks"), true)) %>" target="_blank">
<img src="<%# CMSTransformation.getThatThumbnail((int)Eval("FileID"), (int)100, (int)100, (int)0, "genericSquare") %>" alt="" /> <-- Note: I used a custom function to get my thumbnails
</a></td>
<%# ((DisplayIndex % [column size] + "").Equals("[column size - 1]") ? "</tr>" : "" %>