Inside engine heads, there are intake and exhaust valves that are actuated by the cam directly or through push rods depending on designs. The cam action opens the valves to allow fuel/air in and exhaust out. The springs apply the return tension to the valves to close them as the cam rotation cycles through so (hopefully) the valves don't hit the top of the piston(s) when the pistons are at the top of there stroke. With a bad spring(s) there is less compression during the entire cycle, there can be excessive burning, carbon build-up, and more issues. Plus you risk hitting the piston and bending a valve or holing the piston and dynamiting the engine completely. This is where you have options though, you HAVE to get the spring(s) replaced, BUT is it worth looking at a cam and spring swap at the same time for better performance or pulling the head and porting it along with the springs/cam options. You "could" consider a new head, sending yours to a machine shop or get an aftermarket system. Downside "can" be putting more stress on the low end of the engine, obviously will need a new gasket set, etc.
So, the worst case is destroying your engine, especially if it's an interference type to start, would be similar to breaking a timing belt then. But yeah, don't be running it far if at all and NO high RPMs at all. The ONLY way I would even suggest that is with a NON-Interference engine, so if the spring let go it won't cause deeper damage. Best advise is sideline it now and get it fixed.
EDIT: DUUH, I forgot to include that BMW with VANOS and Ford with VVT solenoids and cam phasers and some Honda valve systems DO control the valve timing or positioning or what opens up on multi-cam systems, so the actual Rocker arm or assembly can be detented or advanced. Still need to fix it, with the Ford truck 5.4 3 cam heads you can test the solenoids at least by just unplugging them and they default into a non-advanced mode.