Apache, properly called the Apache HTTP Server, is software that turns a computer into a web server. When someone types your address into a browser, Apache is one of the programs that can catch that request and send back the right page, image, or file. It has powered a big chunk of the internet since the 1990s.
A good way to picture it is a receptionist at a busy office. Visitors come to the front desk and ask for someone or something, and the receptionist figures out where to send each one and hands back what they came for. Apache does that for web traffic: it takes incoming requests and decides which file or web server response to return.
Apache is free and open-source, and it runs almost anywhere, especially on Linux. It became famous as the A in the classic LAMP stack, alongside MySQL, PHP and Linux, which is what a huge number of WordPress sites still run on today. One reason it stayed popular is the .htaccess file: it lets you change settings per folder without restarting the whole server, which is handy on shared hosting where you do not control the full machine. For a long time it was the default choice, though many teams now compare it with Nginx depending on how much traffic they expect and how they want to configure things.
At TopDevs we pick the web server that fits the project, and when a setup already runs on Apache we keep it tuned and secure rather than rebuilding it for no reason.
