The phrase "diurnal range" refers to the difference in temperature between the warmest and the coolest points of a single day. It tells us how much the temperature changes within a day. Full definition
Very simply, one could hypothesize that site impairment would hit the Tmin record more severely than the Tmax record, narrowing the diurnal range, and raising the mean, consequently. (realclimate.org)
An increased greenhouse effect due to humidity, CO2, aerosols or clouds is expected to produce a relative increase of the minima with respect to the maxima and a decrease of the diurnal range. (skepticalscience.com)
The physical mechanism responsible for the observed decrease in the diurnal range is not known. (realclimate.org)