Security IntelliSense extension is part of Secure DevOps Kit for Azure. The extension is mainly targeted towards providing quick and inline security suggestions and fixes for Azure related C# source code and XML config files. The extension also targets web projects and crypto usage.
The extension is in preview. Additional improvements and features are in pipeline.
After installation, you can try the extension against the sample project from here - https://github.com/azsdk/azsdk-secintel-samples to understand its capabilities.
To know more about the Secure DevOps Kit for Azure, please visit - https://github.com/azsdk/azsdk-docs