They're a web server and reverse proxy that uses the Nginx open-source software as its base, allowing users to create custom configurations for their websites. Their main purpose is to provide high-performance load balancing and reverse proxying capabilities without having to manually configure individual servers or manage multiple instances of different software. They offer flexible options such as SSL certificates, caching, and error handling features which enable businesses to scale their web presence effectively. Additionally, Nginx's robustness allows it to handle high traffic volumes with ease, making it an excellent choice for large-scale applications and services.