It is possible that the site is referring to a common behavior of text areas where holding <kbd>Shift</kbd> and hitting the cursor keys moves the cursor and toggles the highlighting of the characters that the cursor moves over. It is also common for holding <kbd>Ctrl</kbd> and hitting the cursor keys to make the cursor jump over words. Holding down both <kbd>Ctrl</kbd> and <kbd>Shift</kbd> would cause the cursor to highlight text and jump over words.

`{item1,item2}` is somewhat arcane notation for displaying multiple possibilites. <kbd>Shift</kbd>{<kbd>Left</kbd>,<kbd>Right</kbd>} expands to <kbd>Shift</kbd><kbd>Left</kbd> and <kbd>Shift</kbd><kbd>Right</kbd>.