A “hook” is an interesting part of your story meant to grab the reader's attention. Much is said about the elusive hook, but with practice, you can quickly become adept at getting the reader's attention.
Using a hook merely means that you start your piece in a way that will engage the reader and make him or her want to read more. A hook can be a piece of dialogue or description. The key is to make sure it's interesting. Here are three different takes on starting off with an action hook:
Harold blinked as the old lady drew a gun out of her purse and pointed it at the convenience store clerk.
“Give me all your money,” the little old lady said, waving a gun at the convenience store clerk.
The convenience store clerk looked on in disbelief as the little old lady drew a gun from her purse and leveled it at his chest.
The trick is to start with the action. How you actually start will depend on your objective and on your viewpoint. When you're starting a story, don't spend a lot of time establishing setting or character. You want the reader to want to read your story, and expounding information at the beginning of your piece is a sure way to lose the reader's interest. If you start with the action, you will pique the reader's curiosity; you can then work in setting and character after you've whetted the reader's appetite for more.
Likewise, whether in a piece of nonfiction or a letter, the temptation is to provide all sorts of details — or “backstory” — before getting to the point. Writing that states its objective (the action) up front is more effective.