The rules aren't talking about a literal computer database. (Well, you could, but don't try unless you're an Oracle programmer by day or something.)

It means, if you're working in print, a three-ring binder. Or a card catalog. On a computer it can be as simple as a folder where you keep the documents for your work. But it implies some level of organization.

I use Scrivener, so in my case, it means my Scrivener project.

I strongly recommend you go with something you already know how to use. Don't try to pick up new organizational skills in October or November; save that for January, if you find whatever system you use now doesn't work out.

Before I discovered Scrivener, I was experimenting with Freeplane (a very good mind-mapping software) and considering trying to use a wiki database. But Scrivener already had the organizational tools I needed, so for me it made better sense to invest in that. Just to be clear though; I did this AFTER NaNoWriMo.
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Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown