Say you're reading a document in Word and you stumble across a word or topic and you're not entirely sure what it means. Smart Lookup is the answer. Highlight the word then right-click on it and select 'Smart Lookup'.
A pane will pop up on the right-hand side. First, you'll see some great research links providing you with more information about the word. Over to the right, you'll see the Define tab, which you can click on if you just want a definition of the word.