A DBA is simply a Doing Business As.

All it does is create a pointer saying, in effect, "this entity doesn't really exist, it's another name for this other entity." Sort of like DNS for the real world

So you create, for example, widgets, LLC, and a DBA called Janky Enterprises and another DBA called Shiftless Unlimited. You register with the state (or county, depending on your local laws) and say "Shiftless Unlimited is a DBA (otherwise known as a trade name) for Widgets, LLC. or "Janky Enterprises, DBA Widgets LLC"

Legally, Shiftless, Unlimited doesn't exist, but if anyone goes after it, they will check the records and find out that it is another name for Widgets, LLC.

By the way, in many states, individual proprietors can do DBAs legally, so, for example, you could have Joe Blow doing business as John Smith, which would allow Joe Blow to sign contracts and enter into agreements as somebody other than who he is, shielding his true identity at least a little bit. Of course, if he gets sued, then it will be Joe Blow, and not John Smith, who gets sued.