Here's a simple explanation:
DAC is the acronym for "Digital to Analog Converter" -- in short, it takes the digital signal from the CD or music file and converts it into an analog signal that can be amplified and played through your speakers -- in other words, an analog signal just like you'd get from a radio tuner, phonograph or tape deck.
Now ask yourself, do all tuners, phono setups and tape decks sound exactly the same? To most who are serious about their hi-fi setup, the answer is no. That's why you have forums like this where people discuss things in their search for the gear that best suits their preferences within their budget.