Kings who won wars would put their feet on the necks of those whom they had defeated. See how "put ... under his feet" is translated in 1 Corinthians 15:25. AT: "God has completely destroyed all of Christ's enemies" (See: rc://en/ta/man/translate/figs-idiom)
This can stated as active. AT: "God has made all things subject to Christ" (See: rc://en/ta/man/translate/figs-activepassive)
This can stated as active. AT: "the Son himself will become subject" (See: rc://en/ta/man/translate/figs-activepassive)
In the previous verses he was referred to as "Christ." AT: "Christ, that is, the Son himself"
This is an important title that describes the relationship between Jesus and God. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)