Drupal’s database and code depends on having a well-organized file structure. MAMP, and its Windows and Linux counterparts, essentially turn a single folder in your computer into a miniature development server. This means that all of the sites you develop in MAMP will be subfolders of that main folder (e.g. /MAMP/my-crazy-awesome-site). Once you have MAMP installed, it’s important to make sure you set the location of the main folder to something that makes sense for your filesystem, and to back up that folder regularly. I like to keep my MAMP folder in a Dropbox (http://getdropbox.com), which allows me to sync my site files in the cloud and access them from any computer I’m on.