This task concern constraints on one table only.
Two indexes i1 and i2 are called constraint correlated if some of their parts are contained in the same constraint.
Consider table lineitem from dbt3 benchmark.
Index i1: on lineitem(l_shipdate)
Index i2: on lineitem(l_receiptdate)
And let's add constraint c on lineitem: l_shipdate < l_receiptdate
(can be easily checked that it is true for the lineitem table)
As i1 l_shipdate and i2 l_receiptdate both participate in c, indexes i1 and i2 are constraint correlated.