This bypass adapter screws into your head near the thermostat housing, facing downward towards your water pump. The usually corrugated bypass hose connects to this fitting.
These are sometimes badly corroded and it can therefore be difficult to get a good seal with your bypass hose. If you are removing the radiator or changing your water pump, it is very smart to change the bypass hose, and while you are at it, make sure you inspect this fitting. Keep one these handy so you can replace the fitting if needed.