Does this do anything at all. I have read so much that doing more reps at lower weight gives better tone and definition. So you would have to do about 15 to 20 reps with 5 pound weight is this right? I have never tried this before I have been lifting weights for two years but started out heavier and lower reps and have just contunied on at that.

I have read so much that doing more reps at lower weight gives better tone and definition.
What you have read is wrong.

Definition means less bodyfat, which has nothing to do with weight training, and everything to do with diet and cardio.

low weight is only good for making sure that you do the excercise with the proper form or warming up

My trainer sometimes makes me use less weight to get me to focus on using my body right...retracting my blades better or not using my biceps when I should be working on something else... but not something as low as 5lb weights and do more reps. A pregnant woman may be encouraged to use less weight and do a few more reps.