DPI (Dots per inch) is a standard used to measure mouse sensitivity. The higher your mouse's DPI, the faster your cursor moves.
So if you have a large screen monitor, a higher DPI will be very useful. You will reach from one end of the screen to the other with less hand movement. You can also adjust the DPI level for specific tasks like in-game targeting, photo editing, drawing, etc.
So if you're a hardcore gamer and planning to buy a gaming mouse under ₹1000, then make sure it has at least 6400 DPI.