An origin group is a collection of origins that can handle similar types of requests. You need a different origin group for each application or workload that is different.
In an origin group, you create an origin for every server or service that can serve requests. If your origin has a load balancer, like Azure Application Gateway, or is hosted on a PaaS that has a load balancer, then the origin group only has one origin. Your origin takes care of failover and load balancing between origins that Front Door doesn't see.
For example, if you host an application on Azure App Service, how you set up Front Door depends on how many application instances you have:
Single-region deployment: Make one origin group. In that origin group, make one origin for the App Service app. Your App Service app might scale out across workers, but Front Door sees one origin.
Multi-region active/passive deployment: Make one origin group. In that origin group, make an origin for each App Service app. Set each origin's priority so that the main application has a higher priority than the backup application.
Multi-region active/active deployment: Make one origin group. In that origin group, make an origin for each App Service app. Set each origin's priority to be the same. Set each origin's weight to control how many requests go to that origin.
To learn more, see Origins and origin groups in Azure Front Door.