? is a don't care value - i.e. it will match either 0, 1 or z. So the 3'b1?? case will occur whenever the first bit is 1, no matter what the other two bits are.
The circuit is basically a 4:1 multiplexer, selecting one of b0,b1,b2,b3, with an added select bit on a
that forces the output to 0, no matter what the other two select bits are.