Water temperature during coffee brewing determines the extraction yield, the total dissolved solids, and chemical profile of the coffee. All these parameters will affect the aroma and flavor of the final cup.
Some compounds in the coffee bean are only extracted at very high temperatures. These compounds give coffee a bitter taste, and they are undesirable in the final cup.
If water is too hot or the extraction time is too long, coffee is over extracted. Over extraction will cause coffee to be bitter and too strong.