For instance... I go to you blog. You recommend a book in your widget. I click the link and decide I don't like the book but want to see other books in that category. I search Amazon from that same window. I find a book I like. I add it to my cart. You just got credit for that referral b/c I never closed the window.
Now, suppose I close the window, come back in two days by simply typing Amazon.com in the address box. I complete the transaction for the book I placed in my cart two days ago. You get referral fee for that purchase.