The forward slash isn't just a key on your keyboard, but a transporter into the search box in Gmail, Google Calendar, Twitter and many web apps all over the internet. How do you know if it works? Just go to some web app that has a search box and press forward slash.
If you're magically taken to the search box, you'll be able to start searching. There isn't much more to say about it than that, but if you want a lengthy explanation on this simple little shortcut, check out the video above.
The 40 most important Google Apps keyboard shortcuts [Tech Republic]