what does it mean if you balls drop?
temperature changes, your balls raise, lower, bigger, smaller
~*~Smile, it's the second best thing you can do with your mouth.~*~
Sometimes this terminology is also used when describing the initial phases of puberty when a males testicles "drop" into the scrotum as things mature. This is when the scrotum begins to hang lower and the boy begins to take the form of a young man.
To add to what lance said, its slang for reaching puberty. However, its not correct terminology, because the testicles never really do drop during puberty. While the testicles are usually inside the body as an unborn baby, they drop into the scrotum shortly before you are born, or shortly after you are born. Once they are in the scrotum, they should stay there the rest of you life. They don't 'drop' during puberty.
What does change during puberty is the scrotum (the sac that holds your balls) changes and becomes darker, more wrinkly, and elastic, and can give the illusion that your balls are hanging lower than before, although they really aren't. The movement of you balls up and down is dependent on temperature changes, and that function works both before puberty and after puberty.
Also, from beginning to end of puberty, the testicles grow about 10 times their original size, which is a huge increase in size. So they look bigger and seem to hang lower from that as well, and is probably what coined the term "have your balls dropped yet"
<i>My avatar is a picture of my own sperm under a microscope. Pretty cool huh?</i><br />Drugs are garbage. I'm high on my own testosterone.