When you click on the button it will begin to check the spelling of all the words. When it finds a word that it thinks is spelled wrong it gives you several options to fix it.
One of those options is to choose to ignore the word this one time or all the times it comes across the word on the page. This is used when there is a special word that the spell checker just doesn't recognize but you know is spelled the way you want (i.e.. a proper name).
