BuddyPress.org

It is a translation function. Wrapping ‘Blog Posts Feed’ in _e() ensures that it appears in the .po file that translators use to create language packs. The ‘buddypress’ argument ensures that it is associated with the ‘buddypress’ textdomain (since potentially the same string might be translated in different ways when part of different software packages).

The difference between __() and _e() is that _e() echoes the string to the screen.