
And why is it needed?
The security question is how you lock your account so that only you can access it. The main use is for the time that you forget your password. Before it can be given to you, you must answer the security question correctly.
Suppose this wasn't there. Anyone could say they forgot the password for your account. If they're just given the password, then they would have access to your account. We don't want that (nor do you). Thus, if you forget your password and click on the Forgot Password? link to get access to your account, you must answer the security question -- that you make up.
Let's say that the question you enter is: What was your first dog's name? That sounds like a good question that only you would know the answer. You then enter Max as the answer. Then when you forget your password, you will be asked: What was your first dog's name? If the answer is entered as anything other than Max, your account is locked and access is refused.
You want to make your question and answer something no one would be able to guess, but you remember it easily. Your first dog's name is actually not so good as many people could know the name. And it could even be found by searching the Internet.
Here's a really good question: How old were you when you had braces put on your teeth? You know the answer but it's unlikely that someone trying to break into your account would know.
Here's a list of questions that have been used in the past:
As you can see, some are good, and some are not so good. It's up to you to create a good question and answer.