Web Application Proxy is a service in Windows Server 2012 R2 that allows end users to access applications from outside the corporate network on any device.
According to Microsoft, Web Application Proxy pre-authenticates application access with Active Directory Federation Services (ADFS) and also provides reverse proxy functionality. Web Application Proxyshould always be deployed with ADFS, but it can also be deployed with a VPN in an organization's Remote Access deployment.
Web Application Proxy works in two main ways. It makes organizations capable of giving end users outside of an organization selective access to applications that run on servers in the organization. It also serves as a barricade between corporate applications and the Internet. By using ADFS, the service ensures that only users with authenticated and authorized devices can access corporate applications.