LAMP stack is an open source software bundle used to get web servers up and running. It is software that is needed for literally every web service out there.

The acronym "LAMP" stands for Linux, Apache, MySQL, and PHP.

The reason it is called it a stack is because each level derives off it's base layer. The Operating System (Linux) is the base layer. Then comes Apache - the web daemon that sits on top of the OS. Then comes database daemon that stores all the information served by your web daemon. And finally PHP is used to drive and display all the data, and allow for user interaction.

Besides, P, depending on who you talk to, also stands for Perl or Python, but in general, it is assumed to be PHP.

This what LAMP is.