Originally Posted by 98CoupeV6
Yes, that bottle is your reservoir. It's not very important (mine has been empty for extended periods of time without overheating or damage to the system), but it's a backup incase a lot of your coolant suddenly evaporates or leaks.
He is correct. It's just there to compenate to variations in coolant temperature. It's often called just an overflow. as the coolant gets warmer, it expands and fillis the resevoir a littel more. As it cools, it siphons fluid back into the radiator using he little rubber tube. It's best to keep it near the "low" mark.
THe resevoir also makes the system mostly self bleeding. It will pruge most of the air in the cooling system to the resvoir after you change the coolant.