Water represents approximately 98.5% of the coffee brew, so it's easy to understand why the quality of the water used for coffee preparation is very important.
No matter how good the quality of the beans, bad water = bad coffee. Hence, it is very important to filter out any chemicals added for water treatment, particularly chlorine and other unwanted ingredients and minerals.
Water can be very complex, specifically when it comes to measuring water’s “hardness” or mineral content. The harder the water, the more minerals it contains. Water, being a solvent (something that dissolves things), can only dissolve as much coffee solubles as it has available "room" to dissolve. In other words: If the water is too hard, it will dissolve less coffee than the same volume of soft water would. Water should contain between 100 and 200 ppm (parts per million) dissolved minerals for best results. Do not use distilled water or reverse osmosis water as it has no mineral content and will over-extract the coffee and is hard on your brewing equipment.
Don’t fret, preparing your water for brewing at home does not need to be elaborate or complex, a simple “Brita” type filter (available at most home stores or grocery stores) can accomplish a great deal in removing unwanted ingredients and stabilizing the mineral content in your water. Of course, like with most things water filtration systems range in price from a few bucks for a Brita type filter, to thousands of dollars for a permanently installed water filter and softener system for commercial applications.