It's a bypass valve for the heater core. MOST of the time (except for when the AC is set on HI and when the car is warming up) it's open and allows coolant to flow through the heater core. Bypassing it will just keep coolant running through the heater core even when the car is still warming up and when the AC is set on HI. You probably won't notice a big difference except that it might take your car another minute or two longer to warm up.
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