"Christ is only functionally subordinate to the Father, not ontologically subordinate ... Evangelicals strongly affirm the ontological equality of Son with Father. Yet it is difficult to find doctrinal statements—either in churches or in seminaries—in which the Son is said to be functionally subordinate to the Father. Yet John 14:28; Phil 2:6-11; 1 Cor 11:3; 15:28 all plainly teach the eternal subordination of the Son (John 14 and 1 Cor 11 speak of his present subordination; Phil 2 speaks of his subordination in eternity past; and 1 Cor 15 speaks of his subordination in eternity future). Since these same books strongly affirm the ontological equality of Son with Father, the subordination in view must be functional." 
In other words, to say that the Father and the Son are "ontologically equal" means that their being is not to be distinguished, thus preserving the doctrine of monotheism which is essential to the larger doctrine of the Trinity. Furthermore, to say that the Son (or Jesus) is "functionally subordinate" means that Jesus is subordinate according to his functions in the Godhead.
Cyril of Alexandria (376-444) said that,
"yet we are not ignorant that while he remained God, he also became man and subject to God, according to the law suitable to the nature of the manhood. But how could he become the God or Lord of himself? Consequently as man, and with regard to the measure of his humiliation, it is said that he is equally with us subject to God; thus he became under the Law, although as God he spake the Law and was the Law-giver." ( Epistle of Cyril to Nestorius with the XII Anathematisms)