Timing may refer to spark timing or valve timing, but usually refers to spark timing.
Timing is measured in degrees before TDC.
More timing means the spark plug will ignite the intake charge sooner. Modern ECUs adjust timing continuously based on maps? that take into account the volume of air entering the engine and the load? on the engine.
Too little timing leads to reduced torque and horsepower.
Too much timming can lead to detonation and even a blown engine.