Sunteam Paul gets it! That wasn't so tough now, was it?
So the tough way: An intuitive person would see the guy on the left sort of greeting the guy on the right (and vice-versa), so that's a "Yo", and they would probably call the other a brother, hence "Bro".
The easy way is this: The guy in the left picture is named "Ne-Yo", and the right picture shows Barack Obama's half-brother.
So, there were two different ways to arrive at Yo, Bro! Was it too difficult?