The audio conferencing pin allows you to start a MS Teams meeting by calling in rather than using the MS Teams app from a computer. This effectively makes MS Teams behave like a simple audio conference bridge.
In Outlook, click on the schedule icon, then click on "New Teams Meeting". In the resulting blank meeting invitation there will be a link in the meeting options. CTL-CLICK that link. Login if prompted with your email address and computer password.
You will then see this presented. Click on the Reset Pin button. A random pin will be generated for you. Ensure that you remember your pin.