Cross-Origin Resource Sharing (CORS) rule in Azure Blob Storage is to control access to your storage resources from web pages hosted outside your domain.

When you enable CORS for your storage account or container, you can define a set of rules that specify which domains are allowed to access your resources and what types of HTTP requests are permitted. By default, web browsers restrict cross-origin HTTP requests, but by using CORS, you can allow web pages from other domains to make authorized requests to your storage resources.

CORS can be useful in scenarios where you want to enable browser-based uploads or downloads of files to/from your storage account, or when you want to allow a web application hosted in a different domain to access your storage resources.

In Short, the CORS rule in Azure Blob Storage enables you to control which web pages are allowed to access your storage resources from outside your domain, and what types of requests they are authorized to make.