What is a selective constraint?
It is selective pressure that determines and limits the number of neutral/beneficial mutations that can take place. The more constrained a locus is under selection, the easier it is to generate deleterious mutations and the harder it is to generate beneficial ones; this can be reflected in low substitution rates at such loci in populations with time. 

One widely used method to measure this is to compute the Ka/Ks ratio to infer if a locus is selectively constrained or not.