Coffee is almost all water.
Its minerals help extract flavor. Good-tasting, filtered tap water is a fine start.
What it is
A cup of filter coffee is about 98% water, so the water's taste and minerals matter.
Why it matters
Minerals like magnesium and calcium help extract flavor. Too many make coffee flat and chalky; too few make it sharp. Chlorine and off-flavors go straight into the cup.
In practice
If your tap water tastes good, start there; a simple carbon filter helps. Avoid distilled water, and very hard water that leaves scale in the kettle.