Well, your real name was given to you when you were just a newborn baby, before you developed your personality, talents, and looks. Because of this, your name actually says more about your parents than it does about you. They may have named you after a relative, or a famous person they admire, or even because they just liked the sound of a name.
But your other name, the one your friends call you, probably comes from something specific about you. If you're tall, people might call you Stilts. If you're smart, you may be called Braniac or Einstein. If you're good at basketball, you might get the name Hoops.
Nicknames like these are a way of giving meaning to a name, and letting a name describe the person who has it. The name Bill doesn't really tell you anything about a person, but if Bill is also known as Flash, you can guess that he's a fast runner, or that he has a flashy personality.
In the next section, we look at Good Nicknames.