Azure Active Directory's Application Proxy provides secure remote access to on-premises web applications. After a single sign-on to Azure AD, users can access both cloud and on-premises applications through an external URL or an internal application portal.
Azure AD Application Proxy is: Secure. On-premises applications can use Azure's authorization controls and security analytics. For example, on-premises applications can use Conditional Access and two-step verification. Application Proxy doesn't require you to open inbound connections through your firewall.
Simple to use. Users can access your on-premises applications the same way they access Microsoft 365 and other SaaS apps integrated with Azure AD. You don't need to change or update your applications to work with Application Proxy.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/app-proxy/application-proxy