DPI, which is acronym for Dots per Inch, generally measures the sensitivity of a mouse. Higher mouse DPI means the mouse cursor can further navigate along the screen. Any mouse DPI with a much higher setting can detect and react to smaller movements.
Your average cheap mouse today uses at least 1600 DPI, which is more than enough. But the importance of DPI matters greatly when it comes to gaming. See, if you have a MacBook with Retina display which have 300 pixels per inch, you more than likely need a much higher DPI. Gaming though especially if you're engaging in first person shooting, the pros prefer a much lower DPI setting which should be at 800. Reason being you'll have precise control of movement. Often times when you have a low DPI setting you'll see gamers with enormous size mouse pads like the one in the image below, this enables full movement of the arms when playing first person shooting games.
If DPI really matters to you, you should try different DPI settings. You have to take into account that each Mouse have different settings, and displays have different screen resolution, and of course there are different operating systems. How you hold and handle a mouse can be habit forming, we even have different motor skills where some individuals have a much quicker slight of hand. In the end, it all comes down to comfort, how the mouse feels in the hands.
So the question that begs an answer is do DPI matter? Especially when it comes down to gaming? The answer to that question in short is no. In most cases when peripheral manufacturers post DPI numbers its basically a marketing ploy to bait the consumer into a purchase.