Azure Application Gateway is a load balancing solution provided by Microsoft Azure. This web traffic load balancer works on Layer 7 of the OSI model and enables you to manage traffic for your web applications. Unlike traditional load balancers that operate at Layer 4 and route traffic based on source IP address and port, Azure Application Gateway makes routing decisions based on additional attributes of an HTTP request such as URI path or host headers.
Source: What is Azure Application Gateway? Step-by-Step Tutorial (intellipaat.com)
While both Front Door and Application Gateway are layer 7 (HTTP/HTTPS) load balancers, the primary difference is that Front Door is a non-regional service whereas Application Gateway is a regional service. While Front Door can load balance between your different scale units/clusters/stamp units across regions, Application Gateway allows you to load balance between your VMs/containers etc. that is within the scale unit.