It's a correction on your true altitude, as the lower temperature deviates the indicated ALT from your true ALT in a unsafe direction (remember that your altimeter is calibrated to ISA conditions only).
It's about 4% for every 10 degrees of isa deviation.
It's important to apply the correction (if not done by the ATC) from at least MSA down to the minima.
check this:
http://www.smartcockpit.com/data/pdf...Operations.pdf
All the best.