MAMP is basically a solution stack to develop your PHP Web Applications. It comes for Mac as well as for Windows. Here M is for Mac since originally it comes with Mac only, A is for Apache, M for MySQL and P for PHP.
It comes in 2 versions MAMP and MAMP pro which are bundled together. MAMP is a free, local server environment that can be installed under macOS and Windows with just a few clicks. It has all the tools for you local development environment.
MAMP pro has different other features like you can create separate host for each of your web project, you can run each of your virtual hosts with its own PHP version, with it's cloud feature you can copy your hosts to dropbox and there are many other features.
Step 1: Visit mamp.info
Step 2: Click on download and select your operating system
For detailed steps you can follow video.