A car’s “timing” is the coupling of the camshaft (opens and closes valves) to the crankshaft, which rotates as the pistons go up and down in the cylinders. The two are connected with either a belt, chain or gears. A belt that slips can cause a mismatch between the cam and crankshaft, resulting in the engine not firing correctly. A jump in timing can also be the result of other issues.
If a metal timing chain or belt has jumped it is usually obvious, especially with a chain because it will start rattling. However, I have seen cases where a belt has jumped without any rattling. Regardless, a jumped belt or chain is not good and should be fixed ASAP. Driving a car with a jumped timing can cause catastrophic damage to the engine. The spark that fires each cylinder may not be in the correct position, which can lead to burning up or even destroying the valves.
If the chain has jumped, I recommend turning off the ignition and not cranking the engine at all. If the belt or chain is still on, you can try turning the key to Run but not all the way to Start. If it starts and the sparks are in the right place, then you can slowly start cranking it. If it keeps jumping then I would suggest having the belt or chain replaced, but without any driving to prevent further damage.