Earlier today I bought some blue headlights, and while installing them I unbolted a part, my friend called it the ECG valve. Regardless, it had a very hard plastic line running from it, towards a little black box towards my tire well.

The line snapped when I moved the valve, when I started the engine, the line was sucking in air. I repaired it with a short section of rubber hose, which seems to be holding fine.

But I'm curious to what this line is, and if for some reason my repair fails, how serious of damage it could cause.

It's hard to say for sure without being there, but it sounds like the vacume control for the EGR valve solinoid. If so, the system is for emmisions (to make the car run with less noxious gasses out the tail pipe). In the old days, before many of the emmision laws that many states have applied, I have heard of people removing the system completely for better engine preformance. Yes, sad but true, your engine actually runs better without emmision systems.