I always heard people saying lifting heavier weights helps build muscle mass while doing more repetitions helps improve muscle definition. I have no trouble understanding the former. But how does one rationalize the latter? In the past, I used to think that doing more repetitions helps burn away fat at the target area, thereby making the muscle looks more define. But after reading answers to this question, it seems the common consensus is that there is no way to target fat loss. So, how does doing more repetitions make the muscle looks more define? Or is this just a myth?
Correct, there is no way to burn fat from a specific location.
Doing more reps of a certain exercise, lets say dumbbell curls, will make your main bicep's "fibers" work more; when the "main" fibers get tired, adjacent fibers are waked up and put to work. So, when you increase the number of reps you force your muscle to activate more fibers in the last few reps. This provokes muscle hypertrophy, your muscle grows (main and adjacent fibers). That is when you start to see a small curved bulk of muscle in your arm. Because men store little fat in the arm, the change is noticeable in a very short period of time; two weeks can be enough.
About your muscles shape, you can't change it, your genetics determine that. However, when you make it grow it is easier to notice its shape, like in the case of the biceps. Unfortunately for other muscles that is not enough; even if you work them really hard, you won't notice them. This is the case of the abdomen. I know guys that have super big and strong abs, but they don't have a six-pack because its all covered with fat.
More reps improve definition, because you activate sleepy fibers; they grow and give a bigger better look to the whole muscle. But, this works only to some extent.
The only thing that will really give you muscle definition is dieting. If you want to see some evidence, look up in youtube "off season bodybuilding". Most of the guys look fat, even though they have huge and nicely shaped muscles. They train like that and then go through a special, low carbs, diet before competitions.
Some of them will even stop drinking regular water and will drink distilled water. This dehydrates your body and gives you even more definition; Of course, this is a dangerous practice and people have died for going to this extremes.