A reverse proxy is a type of proxy server that takes HTTP requests and seamlessly distributes them to one or more backend servers. These are useful as apache load-balancers or a central location for web applications. (For example, if you have a main domain, you can have a subdirectory pointed to a web application located on a completely different server. Eg: https://www.example.com/ as the main domain and https://www.example.com/myapp can be sitting on a completely different server)
Since you are here, you are most likely looking how to enable this feature. We are using Ubuntu server, but the process is similar across all Linux based apache installs.