Refer to figures.
EASA learning objective 022.02.04.01.07 states:'Describe the following errors: static system error; instrument error; barometric error; temperature error (air column not at ISA conditions); lag (altimeter response to change of height).'
EASA learning objective 022.02.04.01.08 states: ‘Demonstrate the use of an altimeter correction table for the following errors: temperature corrections; aircraft position errors.’

Barometric altimeters are calibrated to ISA conditions, therefore the indicated altitude will only reflect an aircrafts true altitude in standard atmosperic conditions. Differences is temperature from ISA (ISA deviation) will cause indication errors. When the outside air temperature is warmer than ISA the indicated altitude will be less than the true altitude because the air is less dense. In colder than ISA conditions, denser air will cause the indicated altitude to be greater than the true altitude which will result is separation from terrain and obstacles being less than expected!
Remeber the phrase: HIGH TO LOW LOOKOUT BELOW!

To mitigate this threat, when an airfield temperatute is 0°C or colder, pilots need to apply a temperature error correction to barometric altimeters for decision heights/altitudes (DH/DA) or minimum descent heights/altitudes (MDH/MDA) and step down fixes inside the fnal approach fix(FAF)

This is done by using an altimeter temperature correction table (see figure) and the error correction is added to the published altitudes/heights.