This refers to the set of browsers that the site needs to work properly on. It includes browsers e.g. Chrome, Firefox, Internet Explorer as well as operating systems and screen resolutions. As of 2014 there are close to 300 devices that access the Internet and hundreds of browser and operating systems. We use the tool Browser Stack which makes the process of testing sites easier. Note due to the amount of browser and operating system combinations, it’s not possible to test every one.
It’s important to familiarize yourself with the browser spec for the project including knowing if the browser/OS/screen resolution you use is on the list before the project starts.