By guessing the correct password, which is where this brute force dictionary comes in. A database, or other encrypted file, has no means of preventing repeat guesses, so you can take as many bites at the apple as you want. With high end GPU clusters you can attempt thousands of guesses per second. If you restrict your guesses to likely answers only (which is the point of the password list) you can break through in a pretty reasonable amount of time, assuming a vaguely common password was used. Of course, if the database or file is encrypted with something like a random and sufficiently long alphanumeric password or similar, that's a whole different story, and your odds of getting in go down significantly.

There are other attacks of course, but those get significantly more complicated and rely on there being some sort of flaw in the encryption scheme to exploit, or you managing to find the password by some other means (sniff it out of memory while the system is live, social engineering, etc).