Density altitude is the pressure altitude corrected for temperature and pressure.
In I.S.A conditions the Density altitude is same as Pressure altitude.
In temperatures above I.S.A conditions or on a hot day, the density altitude goes higher than pressure altitude.
High Density altitude conditions cause engine performance to decrease.
Since the high Density altitude means that the temperature is hot and the air density is low. On take off the aircraft requires a longer Take off run to become airborne.
Low Density conditions cause engine performance to increase.
Since the low Density altitude means that the temperature is cold and the air density is high. It improves the engine performance.