This is the LDAP "compare" operation. It can be used to compare the value of a given attribute with some already known value. In reality this is mostly used to check whether a given person is a member of some group. In such a case you can compare the DN of the user as a given value, with the values in the attribute "member" of an object of the type groupOfNames. If the compare operation fails, this test fails with errorcode 49.