what is deployment slot in azure ?

Deployment slot allows to deploy application in azure with different versions and Urls. We could setup deploy staging version with production but in different slot. And allows to share requests for testing purposes. If the staging sets to fly, easily swap the production and staging version without any file transfer.

What is an Azure resource group?

A resource group is simply an identifier that Azure Resource Manager applies to resources to group them together. This resource group ID allows Azure Resource Manager to perform operations on a group of resources that share this ID.

NB : A resource group cannot include resources from different subscriptions.