It marks a Task List shortcut. You add them with Edit > Bookmarks > Add Task List Shortcut, [Ctrl+E,T] keystroke. That adds an entry to the task list, use View > Task List to display it. Select “Shortcuts” in the combobox. You’ll see a list of all the shortcuts you added. Double-click an entry to jump to the line.
Remove it again with Edit > Bookmarks > Remove Task List Shortcut, [Ctrl+E,T] again.
NOTE: In visual studio 2015 and above, its [Ctrl+K,Ctrl+H]