The doc you're looking for @John K is Monitor apps in Azure App Service > Understand metrics. There a table lists the definitions of the of two metrics.
Requests In Application Queue: The number of requests in the application request queue.
Http Queue Length: The average number of HTTP requests that had to sit on the queue before being fulfilled. A high or increasing HTTP Queue length is a symptom of a plan under heavy load.
Requests Queue gives you the current number of requests the app service is processing whereas the HTTP Queue Length gives the average of those requests being held in the queue on the service plan.