In a reference to the 2008 documentary film Man on Wire, which chronicles Philippe Petit's 1974 high-wire walk between the Twin Towers of New York's World Trade Centre, Obama is portrayed as a tightrope walker, crossing a wire stretched between the invisible Towers.
1. The expression walk a tightrope is used to indicate that someone is in a difficult situation and has to be very careful what they say or do.
2. If you perform a balancing act, you try to deal successfully with two or more people, groups, or situations that are in opposition to each other.
The meaning of this cartoon is open to interpretation, but I think it has to do with the balancing act which Obama has had to perform in order not to seem to be exploiting the memory of 9/11 for political gain following the killing of bin Laden. This hasn't stopped him being accused of hypocricy, however.