The bookmarks feature allows you to mark certain lines, to be able to easily find them again.
You can set or remove a bookmark in a line in two ways:
Moving the insertion cursor to that line, and activate the -> (Ctrl+B) command.
Clicking in the Icon Border next to that line.
Bookmarks are available in the menu. The individual bookmarks are available as menu items, labeled with the line number of the line with the bookmark, and the first few characters of the text in the line. To move the insertion cursor to the beginning of a bookmarked line, open the menu and select the bookmark.
If the Reopen Files option in the Config Dialog is enabled (or if Kate was started by the session manager), your bookmarks will be restored when the document is reopened.