DPI (sometimes called „CPI“) stands for „Dots per Inch“. This value tells you the maximum resolution of a mouse, aka how many single dots it can track inside one inch of your mouse pad. The higher this value is, the higher is the resolution – and with that the maximum speed you can use your mouse without tracking flaws or errors. Roccat’s Savu has a DPI value of 4,000 for example, which means it can track 4,000 single dots per inch. Especially when you’re looking for a mouse for CS:GO you should go for a higher DPI value as fast movements (360 noscope baby!) are quite common.
But: sadly, the DPI value has become a huge marketing tool to fool gamers nowadays. Some mouse manufacturers now state that their mouse can track more than 10,000 DPI, only to make the customer think: „Yeah baby, that has to be the best mouse on the frickin’ planet!“
W R O N G!
Just 4,000 DPI is always enough to track the maximum resolution in-game (also due to the fact that your screen only has 1,920×1,080 pixels normally). Everything above that might sound awesome, but has zero actual use in game as you will never ever use this feature.