Apache is one of the most commonly used open-source web server software. According to W3Techs, roughly one-third of all websites are hosted on an Apache server. The web server handles the communication with the client (usually the browser) to serve the (WordPress) website.
Apache is particularly popular with hosting providers offering (shared) hosting solutions for smaller blogs and sites with not too many users. As the W3Techs reports show, Apache is losing popularity to NGINX, Clourflare Server, LiteSpeed and others.
One of the characteristics of most Apache web servers is the availability of the .htaccess configuration file. Among other things, this file can be used to define redirects and security headers. It can be found in the root folder, where your WordPress files reside. .htaccess files can also be added in other directories to make specific configurations per directory. This is also how Really Simple SSL prevents (PHP) code execution in the public uploads folder.