The web server is the engine that powers the HTML5 client interface in your browser. Pyramid embeds its own internal web server engine for this operation.
If the web server is installed, the engine is instantiated and can accept web requests accordingly. The web server is designed to do little processing - so it should be able to handle significant concurrent requests. However, if needed, the web server can be installed multiple times on multiple servers - into a web farm configuration and load balanced accordingly.
Web Server Options
Depending on the operating system, there are other web server "options" that can be deployed with the internal Pyramid Web Server. Generally, these optional components act as reverse proxy servers that facilitate external URL host headers; SSL encryption / decryption; specific port bindings and load balancing.
Regardless of the options, the other web server layers merely act as proxies to the internal Pyramid web server engine.
Microsoft IIS is an option in Windows deployments. When used, it acts as a 'reverse proxy' server and will allow for the easy use of external host header URLs and SSL certificates and HTTPS binding on the relevant ports (like 443). For IIS to operate, the URL Writer and ARR modules for IIS need to be installed. Once installed the proxy server settings need to be enabled. When choosing IIS as the "Web Server" - these steps are automated for you during the installation.
Note If using the Pulse server engine, the Web Socket protocol must be installed in IIS.
Other Web Server Options
Other web server technologies should work equally well with Pyramid. Details are available here for configuring NGINX.