Unified Standard Format Markers (USFM) are markers that are put in the text to indicate the book, where a chapter starts, the verse, titles and so on. A marker generally starts with a backslash (\) at the beginning of a line of text in the editor.
Elaborate information on the markers is available when an extra documentation package has been installed. Follow these instructions to install that package, and then the elaborate information on USFM is available.
Up-to-date information on the markers is available online in the United Bible Societies Unified Standard Format Marker documentation.
Bibledit supports several of the markers.
By editing the stylesheet of a project, the number of supported markers can be extended. Completely new markers can be added too, but this is not recommended. It is better to stick to the USFM standard. The stylesheet can be edited under Preferences->Stylesheets.