Well, "focus on" is ambiguous - can mean "think very hard about", which is not what is meant here. I suppose that here "target your abs" here means "get your abs, and not other muscles, to do lots of work so that they become more defined".
Yeah I agree with se16teddy. Being a gym fanatic I have heard this phrase a number of times. I believe that it means to solely aim for your abs with that exercise and not any other muscle groups. Some exercises require two or more muscle groups to be worked at the same time.
Well, that means you focus on your abdominal muscles to the exclusion of other muscles. No one in the gym is going to think "focus on" is ambiguous and start wondering if they can think their way to better abs – only in a language forum would you hold out that hope.