Random string generator from JavaScript for kids challenge

I want to ask about the random string generator challenge. So, I got this quiz from JavaScript for kids book from no starch press publisher. I'm newbie and very confused about this challenge, can somebody help me?

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Random string generator

"Make a random string generator. You’ll need to start with a string containing all the letters in the alphabet: var alphabet = "abcdefghijklmnopqrstuvwxyz"; To pick a random letter from this string, you can update the code we used for the random insult generator in Chapter 3: Math.floor(Math.random() * alphabet.length). This will create a random index into the string. You can then use square brackets to get the character at that index. To create the random string, start with an empty string (var randomString = ""). Then, create a while loop that will continually add new random letters to this string, as long as the string length is less than 6 (or any length you choose). You could use the += operator to add a new letter to the end of the string. After the loop has finished, log it to the console to see your creation!"