Here's the context:
The summer of bloodshed following Mursi's ouster frightened away all but the bravest foreign visitors, a heavy blow to an industry that has suffered greatly from nearly three years of upheaval following the 2011 uprising that ousted autocratic president Hosni Mubarak.
"To frighten away" is to make someone afraid enough to leave.
"The bravest" are the most brave.
So, "frightened away all but the brave" would mean that the brave stayed, and those that weren't brave left. And "frightened away all but the bravest" would mean that even some of the people who left were brave and only those who showed the most bravery remained.