It is the ram air intake for the air conditioning system. There are two of them, one on each side.
Here is an image from the ATR-72 FCOM:
ATR-72 Air conditioning System, image from 737ng.co.uk
Details from the same document:
Also, as @Jonathan Walters points out, the intake is a good example of NACA inlet (or NACA scoop) and are found in a number of aircraft, for example Boeing 737.