Stands for "Mac OS X, Apache, MySQL, and PHP." MAMP is a variation of the LAMP software package that can be installed on Mac OS X. It can be used to run a live web server from a Mac, but is most commonly used for web development and local testing purposes.
Apache (or "Apache HTTP Server") is the component used to configure and run the web server. Once installed, Apache enables a Mac to host one or more websites. By configuring and running a local Apache web server, web developers can view their webpages in a web browser without publishing to an external server.
MAMP also includes MySQL and PHP. These two components are common (open source|open source) technologies used for creating dynamic websites. MySQL is a popular DBMS and PHP is a web scripting language. Webpages that include PHP code can access data from a MySQL database and load dynamic content on-the-fly. By installing PHP and MySQL locally, a developer can build and test a dynamic website on his Mac before publishing it on the Internet.
Apache, MySQL, and PHP are open source components that can be installed individually. However, it is faster and easier to install a pre-built "AMP" package like MAMP or MAMP Pro. Both MAMP and MAMP Pro also include a graphical user interface GUI that can be used to manage the local web server.