DB stands for Database Base. What you put into thoes fields is up to you, mostly. The host name is something like mysql.example.com - you'll be setting up a sub-domain. You can use any sub-domain of something you already have here at dreamhost.
for the DB name you'll fill out any unique name you want. However, it has to be unique across your server, not just for your own plan. most people addend the name with their domain name or user name.
Does that make things clearer?