An Azure Resource Group (RG) is a logical grouping of resources or assets within an Azure subscription. This helps you organizing related resources - You can open an RG to see a web app, its associated App Service Plan, and the database that it accesses listed - to remind you that these things are related.
But there are more tangible benefits to Resource Groups.
For example, I create a lot of Azure demos for presentations that I deliver in-person, online, or as part of my GCast show. https://aka.ms/gcast
When I create a demo, I place all assets in the same resource group, which makes it easier to delete all these demo resources when the presentation ends.
Another advantage is the ability to create an ARM for all resources in a Resource Group with a few mouse clicks. This allows you to easily automate the deployment of these resources to a new environment using PowerShell or the Azure CLI. With an ARM, resources are created in the correct order and input parameters allow you to change things like the names and locations of these resources.
Azure also gives you the ability to move everything in a Resource Group from one subscription to another.
Finally, Azure allows you to merge two resource groups.
You can create a new Azure Resource Group in the Azure Portal (either by itself or as part of a resource that will be added to the group); via a REST API; via the Azure CLI; or using Azure PowerShell.
When deciding how to organize your Azure assets, consider keeping together related resources by placing them in the same Resource Group. Also, consider creating a new Resource Group for each of your deployment environments, such as Development, Testing, and Production.