When virtualizing your APIs behind an Azure API Management (APIM) service, you always need to provide the subscription key in your calls. Typically, this is done via the header key Ocp-Apim-Subscription-Key. Your subscription key is always linked to an APIM product which you define in your publisher portal.
Sometimes, you might want to disable working with a subscription key altogether. Although this might not be the best idea, it is easier to use in development scenarios and might sometimes be preferred when working with certain legacy applications which cannot easily be changed.