What is a LAMP stack?

That simply means using Linux, Apache, MySQL and PHP as your operating system, web server, database, and programming language, respectively.

The reason they call it a stack is because each level derives off its base layer. Your operating system, Linux, is the base layer. Then Apache, your web daemon sits on top of your OS. Then your database stores all the information served by your web daemon, and PHP (or any P* scripting language) is used to drive and display all the data, and allow for user interaction.

Don't be overly concerned with the term 'stack'. People really just mean software suite or bundle, but you're using it just fine I am sure as you are.

Linux operating system

Apache web server

MySQL database

and PHP

Reference: LAMP (software bundle)

The "stack" term means stack! That means if you have experience in working with these technologies/framework or not. Since all these come together in a LAMP package, which you can download and install, they call it a stack.