Cans someone explain where the static variable comes from? ::MEDIA

Forgive me for my naivete but I’m modifying a custom extension and I see this variable:

$mediaPath = $fileSystem->getDirectoryRead(MagentoFrameworkAppFilesystemDirectoryList::MEDIA)->getAbsolutePath();

I went through MagentoFrameworkAppFilesystemDirectoryList but can’t find any mention of ::MEDIA. Can someone please clue me in? Thank you! I’m new to M2.