You'll need to activate it first, and here's how:
1. Login to Gmail and click the cog icon in the top right corner, then select settings from the drop-down menu
2. Click "Labs" from the tabs at the top
3. Scroll to the bottom where you'll see "Undo Send" and click enable
4. Click "Save changes"
The next time you compose and send an email you will see the usual status box appear that says "Your message has been sent", only now it'll also say "Undo". This stays visible for roughly 10 seconds. Click this and the original email will reopen and your heart can stop pounding.
Thank me when it happens!