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.
Water temperature, the grind size, and the extraction time can determine the extraction.
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.
If water is too cold of extraction time is too short coffee is under extracted. Under extraction causes coffee to be too weak, with underdeveloped aroma and flavor, and sour.