It's the unique number of your account. You can find it in the settings page of your profile section on our website. The secret key is used for initiating your account when you work with our plugin in IntelliJ IDEA, so rarely you can see the message with the request to input your secret key in order for the plugin to work correctly. In the plugin, the secret key is displayed in file called "CodeGymPlugin.properties".
Also, with the help of a secret key, you can login on our site, so do not give or show your secret key to anyone, unless someone from CodeGym support asks you, in order to help you with an issue.