The LOE placeholder is for you to attach Letters of Explanation.
If you don't have any LOEs you need to attach (or you've put your LOEs in other placeholders), you can leave that placeholder empty.
As Istari mentioned, it is not a mandatory document, you can submit one when you need to explain something to the case officer, so that he assesses your file in light of what you have explained.
Check this thread for more information http://www.canadavisa.com/canada-immigration-discussion-board/letter-of-explanation-samples-its-discussion-related-thread-t411170.0.html