I'm not a huge specialist on this, but I think basically it's like the expression that somebody “throws the dictionary” at you. In the virtual world you just go through all the known forms of words, addresses, symbols, and so on that you can. It's just an all-out blanket attack at every kind of known, recognizable...I won't even say word, but phoneme or something, that is known to these computers.
I can bring people who can explain this and know a lot more about it than I do. My understanding is that if, let's say, your password is “fish” or something, if there's one of these dictionary attacks, you're a dead duck—or a dead fish—because the computer will recognize it.