When you are given a secret message, the message will come with a key, in the format "(red letter)=(green letter). The code itself will be in red letters. You must spin the decoder so that the red letter in the key matches the green letter. Then you must convert all the red letters in the code to green letters accordingly.
You are given the decoder at the very start of the Island by Director D. Throughout the Island, you use it to help you decode messages that will help you progress, such as how you can only access the Control Center with an agent's fingerprint.
Using the decoder kit.
The print sheet of the decoder kit.
It is the first item on Poptropica that you can print.
The decoder can help you progress greatly on your quest, but it’s not a necessary item to have.