it is a surfactant that allows a bit better contact between the coolant and metal surfaces of the motor and radiator.. Most good quality coolants already have a decent additive package already and IMO, not really needed.. if you race and must run a water only mix, then something like "redline" water wetter is a good idea
If it makes you feel better, you can add it but I don't.. Main thing is use a quality name brand coolant.. There are 2 types ethylene/Propylene glycol's
Any aluminum safe mix will work fine (no silicates)
The later has a slightly higher boiling point (was first used in the vette in the US) and is also the "environmentally safe" coolant woohoo! (ooops)
stick to the recommended mix levels too.. Not enough water will hurt the coolants ability to carry the heat away from the motor..