The Zoom Host Key is used to tell the Zoom that you are in fact the host and not a participant when calling in by phone.
1. Open a web browser and go to Zoom.US
2. On the top left corner go to "Sign In"
3. Enter your username and password for your Zoom account and click "Sign In"
4. Scroll to the bottom of the screen and the second last section is called "Host Key"
5. Click on the word "Show" beside the 6 stars as shown here
6. The 6 digit number that appears is your Zoom host key
Your Host Key should not be shared with anyone and should be treated like a password.